diff --git a/dom/bindings/BindingUtils.h b/dom/bindings/BindingUtils.h
index 3e58390c9f..24b47a545d 100644
--- a/dom/bindings/BindingUtils.h
+++ b/dom/bindings/BindingUtils.h
@@ -515,7 +515,7 @@ class ProtoAndIfaceCache
   {
   public:
     PageTableCache() {
-      memset(&mPages, 0, sizeof(mPages));
+      memset(mPages.begin(), 0, sizeof(mPages));
     }
 
     ~PageTableCache() {