diff --git a/js/src/builtin/Map.js b/js/src/builtin/Map.js
index 700ce1a05f45..7a75a1a0e200 100644
--- a/js/src/builtin/Map.js
+++ b/js/src/builtin/Map.js
@@ -75,8 +75,8 @@ function MapIteratorNext() {
     var O = this;
 
     // Steps 2-3.
-    if (!IsObject(O) || !IsMapIterator(O))
-        return callFunction(CallMapIteratorMethodIfWrapped, O, "MapIteratorNext");
+    if (!IsObject(O) || (O = GuardToMapIterator(O)) === null)
+        return callFunction(CallMapIteratorMethodIfWrapped, this, "MapIteratorNext");
 
     // Steps 4-5 (implemented in _GetNextMapEntryForIterator).
     // Steps 8-9 (omitted).
@@ -95,7 +95,7 @@ function MapIteratorNext() {
         // Steps 10.b-c (omitted).
 
         // Step 6.
-        var itemKind = UnsafeGetInt32FromReservedSlot(this, ITERATOR_SLOT_ITEM_KIND);
+        var itemKind = UnsafeGetInt32FromReservedSlot(O, ITERATOR_SLOT_ITEM_KIND);
 
         var result;
         if (itemKind === ITEM_KIND_KEY) {
diff --git a/js/src/jit/MCallOptimize.cpp b/js/src/jit/MCallOptimize.cpp
index cbc881c6c499..bbe1a4786642 100644
--- a/js/src/jit/MCallOptimize.cpp
+++ b/js/src/jit/MCallOptimize.cpp
@@ -321,8 +321,8 @@ IonBuilder::inlineNativeCall(CallInfo& callInfo, JSFunction* target)
         return inlineSubstringKernel(callInfo);
       case InlinableNative::IntrinsicGuardToArrayIterator:
         return inlineGuardToClass(callInfo, &ArrayIteratorObject::class_);
-      case InlinableNative::IntrinsicIsMapIterator:
-        return inlineHasClass(callInfo, &MapIteratorObject::class_);
+      case InlinableNative::IntrinsicGuardToMapIterator:
+        return inlineGuardToClass(callInfo, &MapIteratorObject::class_);
       case InlinableNative::IntrinsicIsSetIterator:
         return inlineHasClass(callInfo, &SetIteratorObject::class_);
       case InlinableNative::IntrinsicGuardToStringIterator:
diff --git a/js/src/vm/SelfHosting.cpp b/js/src/vm/SelfHosting.cpp
index cf43bb29be07..8f3b96a9f456 100644
--- a/js/src/vm/SelfHosting.cpp
+++ b/js/src/vm/SelfHosting.cpp
@@ -2377,9 +2377,9 @@ static const JSFunctionSpec intrinsic_functions[] = {
     JS_INLINABLE_FN("GuardToArrayIterator",
                     intrinsic_GuardToBuiltin<ArrayIteratorObject>, 1,0,
                     IntrinsicGuardToArrayIterator),
-    JS_INLINABLE_FN("IsMapIterator",
-                    intrinsic_IsInstanceOfBuiltin<MapIteratorObject>,   1,0,
-                    IntrinsicIsMapIterator),
+    JS_INLINABLE_FN("GuardToMapIterator",
+                    intrinsic_GuardToBuiltin<MapIteratorObject>,   1,0,
+                    IntrinsicGuardToMapIterator),
     JS_INLINABLE_FN("IsSetIterator",
                     intrinsic_IsInstanceOfBuiltin<SetIteratorObject>,   1,0,
                     IntrinsicIsSetIterator),