diff --git a/src/builtins/strings.rs b/src/builtins/strings.rs
index 546a4a2..dab8a14 100644
--- a/src/builtins/strings.rs
+++ b/src/builtins/strings.rs
@@ -101,12 +101,13 @@ fn indexof(span: &Span, params: &[Ref<Expr>], args: &[Value], _strict: bool) ->
     ensure_args_count(span, name, params, args, 2)?;
     let s1 = ensure_string(name, &params[0], &args[0])?;
     let s2 = ensure_string(name, &params[1], &args[1])?;
-    for (pos, (idx, _)) in s1.char_indices().enumerate() {
-        if s1[idx..].starts_with(s2.as_ref()) {
-            return Ok(Value::from(Number::from(pos)));
-        }
-    }
-    Ok(Value::from(Number::from(-1i64)))
+
+    let pos = match s1.find(s2.as_ref()) {
+        Some(n) => n,
+        _ => return Ok(Value::from(Number::from(-1i64))),
+    };
+
+    Ok(Value::from(Number::from(pos)))
 }
 
 fn indexof_n(span: &Span, params: &[Ref<Expr>], args: &[Value], _strict: bool) -> Result<Value> {
diff --git a/tests/interpreter/cases/builtins/strings/indexof.yaml b/tests/interpreter/cases/builtins/strings/indexof.yaml
new file mode 100644
index 0000000..460f8b0
--- /dev/null
+++ b/tests/interpreter/cases/builtins/strings/indexof.yaml
@@ -0,0 +1,115 @@
+# Copyright (c) Microsoft Corporation.
+# Licensed under the MIT License.
+
+cases:
+  - note: base
+    data: {}
+    modules:
+      - |
+        package test
+
+        v1 = indexof("Hello world", "llo") # valid substring
+        v2 = indexof("Hello world", "hel") # case sensitive
+        v3 = indexof("Hello world", "l") # single character
+        v4 = indexof("", ",") # empty string
+        v5 = indexof("", "") # empty substring and string
+
+    query: data.test
+    want_result:
+      v1: 2
+      v2: -1
+      v3: 2
+      v4: -1
+      v5: 0
+
+  - note: undefined-string
+    data: {}
+    modules:
+      - |
+        package test
+        x { false }
+        y = indexof(x, "")
+    query: data.test
+    want_result: {}
+
+  - note: undefined-substring
+    data: {}
+    modules:
+      - |
+        package test
+        x { false }
+        y = indexof(",", x)
+    query: data.test
+    want_result: {}
+
+  - note: invalid-null-string
+    data: {}
+    modules: ["package test\nx=indexof(null, ``)"]
+    query: data.test
+    error: "`indexof` expects string argument."
+
+  - note: invalid-bool-string
+    data: {}
+    modules: ["package test\nx=indexof(true, ``)"]
+    query: data.test
+    error: "`indexof` expects string argument."
+
+  - note: invalid-number-string
+    data: {}
+    modules: ["package test\nx=indexof(1, ``)"]
+    query: data.test
+    error: "`indexof` expects string argument."
+
+  - note: invalid-array-string
+    data: {}
+    modules: ["package test\nx=indexof([], ``)"]
+    query: data.test
+    error: "`indexof` expects string argument."
+
+  - note: invalid-set-string
+    data: {}
+    modules: ["package test\nx=indexof(set(), ``)"]
+    query: data.test
+    error: "`indexof` expects string argument."
+
+  - note: invalid-object-string
+    data: {}
+    modules: ["package test\nx=indexof({}, ``)"]
+    query: data.test
+    error: "`indexof` expects string argument."
+
+  - note: invalid-null-substring
+    data: {}
+    modules: ["package test\nx=indexof(``, null)"]
+    query: data.test
+    error: "`indexof` expects string argument."
+
+  - note: invalid-bool-substring
+    data: {}
+    modules: ["package test\nx=indexof(``, true)"]
+    query: data.test
+    error: "`indexof` expects string argument."
+
+  - note: invalid-number-substring
+    data: {}
+    modules: ["package test\nx=indexof(``, 1)"]
+    query: data.test
+    error: "`indexof` expects string argument."
+
+  - note: invalid-array-substring
+    data: {}
+    modules: ["package test\nx=indexof(``, [])"]
+    query: data.test
+    error: "`indexof` expects string argument."
+
+  - note: invalid-set-substring
+    data: {}
+    modules: ["package test\nx=indexof(``, set())"]
+    query: data.test
+    error: "`indexof` expects string argument."
+
+  - note: invalid-object-substring
+    data: {}
+    modules: ["package test\nx=indexof(``, {})"]
+    query: data.test
+    error: "`indexof` expects string argument."