diff --git a/NativeScript/runtime/ArgConverter.mm b/NativeScript/runtime/ArgConverter.mm
index bd6956c6..96cded6a 100644
--- a/NativeScript/runtime/ArgConverter.mm
+++ b/NativeScript/runtime/ArgConverter.mm
@@ -883,16 +883,18 @@
 
   ObjCDataWrapper* objcDataWrapper = static_cast<ObjCDataWrapper*>(wrapper);
   id target = objcDataWrapper->Data();
-  if (![target isKindOfClass:[NSArray class]]) {
+  // Treat anything that can report a count and return elements by index as
+  // indexable, not just NSArray (e.g. PHFetchResult, NSOrderedSet).
+  if (![target respondsToSelector:@selector(count)] ||
+      ![target respondsToSelector:@selector(objectAtIndex:)]) {
     return;
   }
 
-  NSArray* array = (NSArray*)target;
-  if (index >= [array count]) {
+  if (index >= [target count]) {
     return;
   }
 
-  id obj = [array objectAtIndex:index];
+  id obj = [target objectAtIndex:index];
 
   std::shared_ptr<Caches> cache = Caches::Get(isolate);
   auto it = cache->Instances.find(obj);
@@ -959,6 +961,16 @@
   ObjCDataWrapper* objcDataWrapper = static_cast<ObjCDataWrapper*>(wrapper);
   id target = objcDataWrapper->Data();
   if (![target isKindOfClass:[NSMutableArray class]]) {
+    // Indexed assignment is only supported for NSMutableArray. Other indexable
+    // collections (e.g. NSOrderedSet, PHFetchResult) reject the write so it can't be
+    // silently dropped into an unreachable JS property on the wrapper. NSArray is
+    // excluded from the throw and falls through to a silent no-op.
+    if ([target respondsToSelector:@selector(count)] &&
+        [target respondsToSelector:@selector(objectAtIndex:)] &&
+        ![target isKindOfClass:[NSArray class]]) {
+      isolate->ThrowException(Exception::TypeError(
+          tns::ToV8String(isolate, "Indexed assignment is only supported for NSMutableArray")));
+    }
     return;
   }
 
diff --git a/TestRunner/app/tests/ApiTests.js b/TestRunner/app/tests/ApiTests.js
index 84834d17..d67b6e5d 100644
--- a/TestRunner/app/tests/ApiTests.js
+++ b/TestRunner/app/tests/ApiTests.js
@@ -22,6 +22,45 @@ describe(module.id, function () {
         expect(res[0].constructor.name).toEqual("NSURL");
     });
 
+    it("indexed access works for non-NSArray indexable collections", function () {
+        // NSOrderedSet is not an NSArray but responds to count + objectAtIndex:,
+        // so obj[i] should resolve the same elements as objectAtIndex(i).
+        const set = NSOrderedSet.orderedSetWithArray(['a', 'b', 'c']);
+        expect(set.count).toBe(3);
+        expect(set[0]).toBe(set.objectAtIndex(0));
+        expect(set[1]).toBe(set.objectAtIndex(1));
+        expect(set[2]).toBe(set.objectAtIndex(2));
+        expect(set[0]).toBe('a');
+        expect(set[2]).toBe('c');
+        // Out-of-range index returns undefined instead of throwing.
+        expect(set[3]).toBeUndefined();
+    });
+
+    it("indexed assignment throws for read-only indexable collections", function () {
+        const set = NSOrderedSet.orderedSetWithArray(['a', 'b', 'c']);
+        // NSOrderedSet is indexable but not writable, so indexed assignment throws.
+        // The index is irrelevant; the whole collection rejects writes.
+        expect(function () { set[0] = 'ghost'; }).toThrowError(TypeError, /Indexed assignment is only supported for NSMutableArray/);
+        expect(function () { set[5] = 'ghost'; }).toThrowError(TypeError);
+        // The rejected out-of-range write left no stray JS property behind, the native
+        // collection is untouched, and reads still resolve native elements.
+        expect(Object.prototype.hasOwnProperty.call(set, '5')).toBe(false);
+        expect(set.count).toBe(3);
+        expect(set[0]).toBe('a');
+        expect(set[5]).toBeUndefined();
+    });
+
+    it("indexed assignment still works for NSMutableArray", function () {
+        const first = NSObject.new();
+        const arr = NSMutableArray.arrayWithArray([first, NSObject.new()]);
+        const replacement = NSObject.new();
+
+        arr[0] = replacement;
+        expect(arr.count).toBe(2);
+        expect(arr.objectAtIndex(0)).toBe(replacement);
+        expect(arr[0]).toBe(replacement);
+    });
+
     it("MethodCalledInDealloc", function () {
         expect(function () {
             (function () {