diff --git a/.changeset/dry-crews-warn.md b/.changeset/dry-crews-warn.md
new file mode 100644
index 00000000..888d5717
--- /dev/null
+++ b/.changeset/dry-crews-warn.md
@@ -0,0 +1,5 @@
+---
+'@trustnxt/c2pa-ts': patch
+---
+
+BMFF: Fix adjustment of extents in iloc box
diff --git a/src/asset/BMFF.ts b/src/asset/BMFF.ts
index ec300247..00230e37 100644
--- a/src/asset/BMFF.ts
+++ b/src/asset/BMFF.ts
@@ -528,7 +528,7 @@ class ItemLocationBox extends FullBox<ItemLocationBoxPayload> {
 
         for (const item of this.payload.items) {
             pos += this.payload.version === 2 ? 4 : 2; // item_ID
-            if (this.payload.version > 0) pos++; // reserved, construction_method
+            if (this.payload.version > 0) pos += 2; // reserved, construction_method
             pos += 2; // data_reference_index
             if (this.payload.baseOffsetSize === 8) {
                 item.baseOffset = (item.baseOffset as bigint) + BigInt(amount);
@@ -543,8 +543,8 @@ class ItemLocationBox extends FullBox<ItemLocationBoxPayload> {
             for (const extent of item.extents) {
                 if (this.payload.indexSize) pos += this.payload.indexSize;
                 if (
-                    (item.constructionMethod ??
-                        ItemLocationConstructionMethod.file === ItemLocationConstructionMethod.file) &&
+                    (item.constructionMethod ?? ItemLocationConstructionMethod.file) ===
+                        ItemLocationConstructionMethod.file &&
                     item.baseOffset === 0 &&
                     extent.offset !== 0
                 ) {