diff --git a/js/src/vm/NativeObject.cpp b/js/src/vm/NativeObject.cpp
index 9aee0559ab37..022aacd85dc1 100644
--- a/js/src/vm/NativeObject.cpp
+++ b/js/src/vm/NativeObject.cpp
@@ -545,9 +545,13 @@ bool NativeObject::willBeSparseElements(uint32_t requiredCapacity,
     return true;
   }
 
-  uint32_t len = getDenseInitializedLength();
+  uint32_t initLen = getDenseInitializedLength();
+  if (denseElementsArePacked()) {
+    return minimalDenseCount > initLen;
+  }
+
   const Value* elems = getDenseElements();
-  for (uint32_t i = 0; i < len; i++) {
+  for (uint32_t i = 0; i < initLen; i++) {
     if (!elems[i].isMagic(JS_ELEMENTS_HOLE) && !--minimalDenseCount) {
       return false;
     }