diff --git a/js/src/jsinferinlines.h b/js/src/jsinferinlines.h
index de4e176d3b2..aa9fbf83844 100644
--- a/js/src/jsinferinlines.h
+++ b/js/src/jsinferinlines.h
@@ -375,7 +375,7 @@ EnsureTrackPropertyTypes(JSContext *cx, JSObject *obj, jsid id)
             return;
         }
         if (!obj->type()->unknownProperties() && !obj->type()->getProperty(cx, id)) {
-            obj->type()->markUnknown(cx);
+            MOZ_ASSERT(obj->type()->unknownProperties());
             return;
         }
     }