diff --git a/dom/base/nsDOMClassInfo.cpp b/dom/base/nsDOMClassInfo.cpp
index 22e387a06da..f15634d0b52 100644
--- a/dom/base/nsDOMClassInfo.cpp
+++ b/dom/base/nsDOMClassInfo.cpp
@@ -6266,6 +6266,11 @@ ResolvePrototype(nsIXPConnect *aXPConnect, nsGlobalWindow *aWin, JSContext *cx,
     }
 
     if (dot_prototype) {
+      JSAutoEnterCompartment ac;
+      if (!ac.enter(cx, dot_prototype)) {
+        return NS_ERROR_UNEXPECTED;
+      }
+
       JSObject *xpc_proto_proto = ::JS_GetPrototype(cx, dot_prototype);
 
       if (proto &&
@@ -6277,6 +6282,11 @@ ResolvePrototype(nsIXPConnect *aXPConnect, nsGlobalWindow *aWin, JSContext *cx,
         }
       }
     } else {
+      JSAutoEnterCompartment ac;
+      if (!ac.enter(cx, winobj)) {
+        return NS_ERROR_UNEXPECTED;
+      }
+
       dot_prototype = ::JS_NewObject(cx, &sDOMConstructorProtoClass, proto,
                                      winobj);
       NS_ENSURE_TRUE(dot_prototype, NS_ERROR_OUT_OF_MEMORY);