diff --git a/src/AbstractApi.php b/src/AbstractApi.php
index 47bd7a2..d36f640 100644
--- a/src/AbstractApi.php
+++ b/src/AbstractApi.php
@@ -8,7 +8,7 @@
 
 abstract class AbstractApi implements ApiInterface
 {
-    const DEFAULT_API_VERSION = '2019-04';
+    const DEFAULT_API_VERSION = '2020-07';
 
     /**
      * Domain of the Shopify store
diff --git a/src/Enum/Fields/CollectionListingFields.php b/src/Enum/Fields/CollectionListingFields.php
index 5537887..72cf833 100644
--- a/src/Enum/Fields/CollectionListingFields.php
+++ b/src/Enum/Fields/CollectionListingFields.php
@@ -2,7 +2,7 @@
 
 namespace Shopify\Enum\Fields;
 
-class CollectionListingFields extends AbstractObject
+class CollectionListingFields extends AbstractObjectEnum
 {
     const COLLECTION_ID = 'collection_id';
     const BODY_HTML = 'body_html';
diff --git a/src/Enum/Fields/InventoryItemFields.php b/src/Enum/Fields/InventoryItemFields.php
new file mode 100644
index 0000000..2456808
--- /dev/null
+++ b/src/Enum/Fields/InventoryItemFields.php
@@ -0,0 +1,30 @@
+<?php
+
+namespace Shopify\Enum\Fields;
+
+class InventoryItemFields extends AbstractObjectEnum
+{
+    const COST                              = 'cost';                               //The unit cost of the inventory item.
+    const COUNTRY_CODE_OF_ORIGIN            = 'country_code_of_origin';             //The two-digit code for the country where the inventory item was made.
+    const COUNTRY_HARMONIZED_SYSTEM_CODES   = 'country_harmonized_system_codes';    //An array of country-specific Harmonized System (HS) codes for the item. Used to determine duties when shipping the inventory item to certain countries.
+    const CREATED_AT                        = 'created_at';                         //The date and time (ISO 8601 format) when the inventory item was created.
+    const HARMONIZED_SYSTEM_CODE            = 'harmonized_system_code';             //The general Harmonized System (HS) code for the inventory item. Used if a country-specific HS code is not available. 
+    const ID                                = 'id';                                 //The ID of the inventory item.
+    const PROVINCE_CODE_OF_ORIGIN           = 'province_code_of_origin';            //The two-digit code for the province where the inventory item was made. Used only if the shipping provider for the inventory item is Canada Post.
+    const SKU                               = 'sku';                                //The unique SKU (stock keeping unit) of the inventory item.
+    const TRACKED                           = 'tracked';                            //Whether the inventory item is tracked. If true, then inventory quantity changes are tracked by Shopify.
+
+    public function getFieldTypes()
+    {
+        return array(
+            'cost'                              => 'integer',
+            'country_code_of_origin'            => 'string',
+            'country_harmonized_system_codes'   => 'array',
+            'harmonized_system_code'            => 'integer',
+            'id'                                => 'integer',
+            'province_code_of_origin'           => 'string',
+            'sku'                               => 'string',
+            'tracked'                           => 'boolean',
+        );
+    }
+}
\ No newline at end of file
diff --git a/src/Enum/Fields/InventoryLevelFields.php b/src/Enum/Fields/InventoryLevelFields.php
new file mode 100644
index 0000000..79a22db
--- /dev/null
+++ b/src/Enum/Fields/InventoryLevelFields.php
@@ -0,0 +1,23 @@
+<?php
+
+namespace Shopify\Enum\Fields;
+
+class InventoryLevelFields extends AbstractObjectEnum
+{
+    const INVENTORY_ITEM_ID         = 'inventory_item_id';          //The ID for the inventory item.
+    const LOCATION_ID               = 'location_id';                //The ID of the location that the inventory level belongs to. To find the ID of the location, use the Location resource.
+    const AVAILABLE                 = 'available';                  //Sets the available inventory quantity.
+    const AVAILABLE_ADJUSTMENT      = 'available_adjustment';       //The amount to adjust the available inventory quantity. Send negative values to subtract from the current available quantity. For example, "available_adjustment": 2 increases the current available quantity by 2, and "available_adjustment": -3 decreases the current available quantity by 3.
+    const DISCONNECT_IF_NECESSARY   = 'disconnect_if_necessary';    //Whether inventory for any previously connected locations will be set to 0 and the locations disconnected. This property is ignored when no fulfillment service is involved. For more information, see Inventory levels and fulfillment service locations. (default: false)
+
+    public function getFieldTypes()
+    {
+        return array(
+            'inventory_item_id'         => 'integer',
+            'location_id'               => 'integer',
+            'available'                 => 'integer',
+            'available_adjustment'      => 'integer',
+            'disconnect_if_necessary'   => 'boolean',
+        );
+    }
+}
\ No newline at end of file
diff --git a/src/Object/AbstractObject.php b/src/Object/AbstractObject.php
index 5cf3b79..8e63eae 100644
--- a/src/Object/AbstractObject.php
+++ b/src/Object/AbstractObject.php
@@ -51,7 +51,7 @@ public function __construct()
         $this->data = array_fill_keys($enum->getFields(), null);
     }
 
-    public function __get($key)
+    public function &__get($key)
     {
         if (!array_key_exists($key, $this->data)) {
             throw new \InvalidArgumentException(
@@ -61,7 +61,7 @@ public function __get($key)
         return $this->data[$key];
     }
 
-    public function __set($key, $value)
+    public function &__set($key, $value)
     {
         if (!array_key_exists($key, $this->data)) {
             return $this;
@@ -73,7 +73,7 @@ public function __set($key, $value)
         }
         if (!is_null($value) && !$this->isValidValue($key, $value)) {
             throw new \InvalidArgumentException(
-                "Invalid type for property '{$key}'"
+                "Invalid type for property '{$key}', should be a ".$this->types[$key]." received ".print_r($value,1)
             );
         }
         $this->data[$key] = $value;
diff --git a/src/Object/InventoryItem.php b/src/Object/InventoryItem.php
new file mode 100644
index 0000000..49edc07
--- /dev/null
+++ b/src/Object/InventoryItem.php
@@ -0,0 +1,44 @@
+<?php
+/**
+ * Shopify\Object\InventoryItem
+ *
+ * Object representing a single Inventory Item for a product
+ *
+ * MIT License
+ *
+ * Copyright (c) Jelle Dijkhuizen 2020
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ *
+ * @package Shopify
+ * @author  Jelle Dijkhuizen <jelle@dijkma.nl>
+ * @license MIT
+ */
+
+namespace Shopify\Object;
+
+use Shopify\Enum\Fields\InventoryItemFields;
+
+class InventoryItem extends AbstractObject
+{
+    public static function getFieldsEnum()
+    {
+        return InventoryItemFields::getInstance();
+    }
+}
diff --git a/src/Object/InventoryLevel.php b/src/Object/InventoryLevel.php
new file mode 100644
index 0000000..840d591
--- /dev/null
+++ b/src/Object/InventoryLevel.php
@@ -0,0 +1,44 @@
+<?php
+/**
+ * Shopify\Object\InventoryLevel
+ *
+ * Object representing a single Inventory Level for an Inventory item of a product
+ *
+ * MIT License
+ *
+ * Copyright (c) Jelle Dijkhuizen 2020
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ *
+ * @package Shopify
+ * @author  Jelle Dijkhuizen <jelle@dijkma.nl>
+ * @license MIT
+ */
+
+namespace Shopify\Object;
+
+use Shopify\Enum\Fields\InventoryLevelFields;
+
+class InventoryLevel extends AbstractObject
+{
+    public static function getFieldsEnum()
+    {
+        return InventoryLevelFields::getInstance();
+    }
+}
\ No newline at end of file
diff --git a/src/Service/AbstractService.php b/src/Service/AbstractService.php
index 6efe4e5..6770aad 100644
--- a/src/Service/AbstractService.php
+++ b/src/Service/AbstractService.php
@@ -2,6 +2,7 @@
 
 namespace Shopify\Service;
 
+use Exception;
 use GuzzleHttp\Client;
 use GuzzleHttp\Psr7\Request;
 use GuzzleHttp\Psr7\Response;
@@ -21,9 +22,9 @@ abstract class AbstractService
      */
     private $lastResponse;
 
-    const REQUEST_METHOD_GET = 'GET';
-    const REQUEST_METHOD_POST = 'POST';
-    const REQUEST_METHOD_PUT = 'PUT';
+    const REQUEST_METHOD_GET    = 'GET';
+    const REQUEST_METHOD_POST   = 'POST';
+    const REQUEST_METHOD_PUT    = 'PUT';
     const REQUEST_METHOD_DELETE = 'DELETE';
 
     public static function factory(ApiInterface $api)
@@ -78,17 +79,37 @@ public function getLastResponse()
 
     public function send(Request $request, array $params = array())
     {
-        $args = array();
+        //Set the empty args array
+        $args = [];
+
+        //If the method is get we need to send the args as a query other wise the args need to be send as json
         if ($request->getMethod() === 'GET') {
             $args['query'] = $params;
-        } else {
-            $args['json'] = $params;
         }
+        else {
+            $args['json'] = $params;            
+        }
+
+        //Load the response in a variable
         $this->lastResponse = $this->client->send($request, $args);
-        return json_decode(
+        
+        //Decode the json string and save to a varibale.
+        //We do need return this derict so we can check for an json error.
+        $return = json_decode(
             $this->lastResponse->getBody()->getContents(),
             true
         );
+
+        //Check if there are any json error.
+        //if there is an error throw an exeption
+        //TOD: make costum exeption.
+        $json_error = json_last_error();
+        if($json_error === JSON_ERROR_NONE){
+            return $return;
+        }
+        else{
+            throw new Exception($json_error);
+        }
     }
 
     public function createObject($className, $data)
@@ -106,4 +127,4 @@ function ($object) use ($className) {
             }, $data
         );
     }
-}
+}
\ No newline at end of file
diff --git a/src/Service/CustomCollectionService.php b/src/Service/CustomCollectionService.php
index 36580ad..1beaba8 100644
--- a/src/Service/CustomCollectionService.php
+++ b/src/Service/CustomCollectionService.php
@@ -100,7 +100,7 @@ public function update(CustomCollection &$customCollection)
      */
     public function delete(CustomCollection &$customCollection)
     {
-        $endpoint = 'custom_collections/'.$customCollection->getId().'.json';
+        $endpoint = 'custom_collections/'.$customCollection->id.'.json';
         $this->request($endpoint, static::REQUEST_METHOD_DELETE);
         return;
     }
diff --git a/src/Service/InventoryItemService.php b/src/Service/InventoryItemService.php
new file mode 100644
index 0000000..b24623b
--- /dev/null
+++ b/src/Service/InventoryItemService.php
@@ -0,0 +1,60 @@
+<?php
+
+namespace Shopify\Service;
+
+use Shopify\Object\InventoryItem;
+
+class InventoryItemService extends AbstractService
+{
+    /**
+     * Retrieves a list of inventory items.
+     * Note: As of version 2019-10, this endpoint implements pagination by using links that are provided in the response header.
+     * Sending the page parameter will return an error.
+     * To learn more, see Making requests to paginated REST Admin API endpoints.
+     *
+     * @link https://shopify.dev/docs/admin-api/rest/reference/inventory/inventoryitem#index-2020-07
+     * @param array $params
+     * @return InventoryItem[]
+     */
+    public function all(array $params = []){
+        $endpoint = 'inventory_items.json';
+        $response = $this->request($endpoint, 'GET', $params);
+        return $this->createCollection(InventoryItem::class, $response['inventory_item']);
+    }
+
+    /**
+     * Retrieves a single inventory item by ID
+     *
+     * @link https://shopify.dev/docs/admin-api/rest/reference/inventory/inventoryitem#show-2020-07
+     * @param int $inventroyItemId
+     * @param array $fields
+     * @return InventoryItem[]
+     */
+    public function get(int $inventroyItemId, array $fields = []){
+        $params = [];
+        if (!empty($fields)) {
+            $params['fields'] = $fields;
+        }
+        $endpoint = 'inventory_items/'.$inventroyItemId.'.json';
+        $response = $this->request($endpoint, 'GET', $params);
+        return $this->createObject(InventoryItem::class, $response['inventory_item']);
+    }
+
+    /**
+     * Updates an existing inventory item
+     *
+     * @link https://shopify.dev/docs/admin-api/rest/reference/inventory/inventoryitem#update-2020-07
+     * @param InventoryItem $inventoryItem
+     * @return void
+     */
+    public function update(InventoryItem &$inventoryItem){
+        $data = $inventoryItem->exportData();
+        $endpoint = 'inventory_items/'.$inventoryItem->id.'.json';
+        $response = $this->request(
+            $endpoint, 'PUT', array(
+            'inventory_item' => $data
+            )
+        );
+        $inventoryItem->setData($response['inventory_item']);
+    }
+}
\ No newline at end of file
diff --git a/src/Service/InventoryLevelService.php b/src/Service/InventoryLevelService.php
new file mode 100644
index 0000000..ae14253
--- /dev/null
+++ b/src/Service/InventoryLevelService.php
@@ -0,0 +1,120 @@
+<?php
+
+namespace Shopify\Service;
+
+use InvalidArgumentException;
+use Shopify\Object\InventoryLevel;
+
+class InventoryLevelService extends AbstractService
+{
+    /**
+     * Retrieves a list of inventory levels.
+     * You must include inventory_item_ids, location_ids, or both as filter parameters.
+     * 
+     * Note: As of version 2019-10, this endpoint implements pagination by using links that are provided in the response header.
+     * Sending the page parameter will return an error. To learn more, see Making requests to paginated REST Admin API endpoints.
+     *
+     * @param array $inventoryItemIds leave emptry if you to see the level of all items at a location
+     * @param array $locationIds leave empty if you want to see the item levels at all locations
+     * 
+     * @return InventoryLevel[]
+     */
+    public function get(array $inventoryItemIds = [], array $locationIds = []){
+        //Set the eandpoint to inventory_levels.
+        $endpoint = 'inventory_levels.json?';
+
+        //First we check what we want to check if there are empty arrays.
+        //If the $inventoryItemIds is not empty but $locationIds is.
+        //We need this $endpoint
+        if(!empty($inventoryItemIds) && empty($locationIds)){
+            $endpoint = $endpoint.'inventory_item_ids='.implode(",", $inventoryItemIds);
+        }
+        //Else the $locationIds is not empty but the $inventoryItemIds is empty.
+        //We need to use this $endpoint
+        elseif(!empty($locationIds) && empty($inventoryItemIds)){
+            $endpoint = $endpoint.'location_ids='.implode(",", $locationIds);
+        }
+        //Else both $inventoryItemIds and $locationIds are not empty.
+        //We need to use this $endpoint
+        elseif(!empty($inventoryItemIds) && !empty($locationIds)){
+            $endpoint = $endpoint.'inventory_item_ids='.implode(",", $inventoryItemIds).'&location_ids='.implode(",", $locationIds);
+        }
+        //Else both arrays are empty.
+        //throw an InvalidArgumentException.
+        else{
+            throw new InvalidArgumentException();
+        }
+
+        //Get the response
+        $response = $this->request($endpoint);
+
+        return $$this->createCollection(InventoryLevel::class, $response['inventory_levels']);
+    }
+
+    /**
+     * Adjusts the inventory level of an inventory item at a single location
+     *
+     * @link https://shopify.dev/docs/admin-api/rest/reference/inventory/inventorylevel#adjust-2020-07
+     * @param InventoryLevel $inventoryLevel
+     * @return void
+     */
+    public function adjust(InventoryLevel &$inventoryLevel){
+        $data = $inventoryLevel->exportData();
+        $endpoint = 'inventory_levels/adjust.json';
+        $response = $this->request(
+            $endpoint, 'POST', $data
+        );
+        $inventoryLevel->setData($response['inventory_level']);
+    }
+
+    /**
+     * Deletes an inventory level of an inventory item at a location.
+     * Deleting an inventory level for an inventory item removes that item from the specified location.
+     * Every inventory item must have at least one inventory level.
+     * To move inventory to another location, first connect the inventory item to another location, and then delete the previous inventory level.
+     *
+     * @link https://shopify.dev/docs/admin-api/rest/reference/inventory/inventorylevel#destroy-2020-07
+     * @param InventoryLevel $inventoryLevel
+     * @return void
+     */
+    public function delete(InventoryLevel &$inventoryLevel){
+        $endpoint = 'inventory_levels.json?inventory_item_id='.$inventoryLevel->inventory_item_id.'&location_id='.$inventoryLevel->location_id;
+        $this->request($endpoint, 'DELETE');
+        return;
+    }
+
+    /**
+     * Connects an inventory item to a location by creating an inventory level at that location.
+     * When connecting inventory items to locations, it's important to understand the rules around fulfillment service locations.
+     *
+     * @link https://shopify.dev/docs/admin-api/rest/reference/inventory/inventorylevel#connect-2020-07
+     * @param InventoryLevel $inventoryLevel
+     * @return void
+     */
+    public function connect(InventoryLevel &$inventoryLevel){
+        $data = $inventoryLevel->exportData();
+        $endpoint = 'inventory_levels/connect.json';
+        $response = $this->request(
+            $endpoint, 'POST', $data
+        );
+        $inventoryLevel->setData($response['inventory_level']);
+    }
+
+    /**
+     * Sets the inventory level for an inventory item at a location.
+     * If the specified location is not connected, it will be automatically connected first.
+     * When connecting inventory items to locations, it's important to understand the rules around fulfillment service locations.
+     *
+     * @link https://shopify.dev/docs/admin-api/rest/reference/inventory/inventorylevel#set-2020-07
+     * @param InventoryLevel $inventoryLevel
+     * @return void
+     */
+    public function set(InventoryLevel &$inventoryLevel){
+        $data = $inventoryLevel->exportData();
+        $endpoint = 'inventory_levels/set.json';
+        $response = $this->request(
+            $endpoint, 'POST', $data
+        );
+        $inventoryLevel->setData($response['inventory_level']);
+    }
+}
\ No newline at end of file