diff --git a/content/events/src/nsDOMEvent.cpp b/content/events/src/nsDOMEvent.cpp
index e0f5dd9941c1..635855b8f408 100644
--- a/content/events/src/nsDOMEvent.cpp
+++ b/content/events/src/nsDOMEvent.cpp
@@ -675,7 +675,7 @@ nsDOMEvent::DuplicatePrivateData()
     {
       nsUIEvent* oldUIEvent = static_cast<nsUIEvent*>(mEvent);
       nsUIEvent* uiEvent = new nsUIEvent(false, msg, oldUIEvent->detail);
-      uiEvent->AssignEventData(*oldUIEvent, true);
+      uiEvent->AssignUIEventData(*oldUIEvent, true);
       newEvent = uiEvent;
       break;
     }
@@ -693,7 +693,7 @@ nsDOMEvent::DuplicatePrivateData()
       nsUIEvent* oldUIEvent = static_cast<nsUIEvent*>(mEvent);
       nsUIEvent* uiEvent = new nsUIEvent(false, msg, 0);
       uiEvent->eventStructType = NS_SMIL_TIME_EVENT;
-      uiEvent->AssignGUIEventData(*oldUIEvent, true);
+      uiEvent->AssignUIEventData(*oldUIEvent, true);
       newEvent = uiEvent;
       break;
     }
diff --git a/widget/nsGUIEvent.h b/widget/nsGUIEvent.h
index 98e1677eff9f..858554ccbb9a 100644
--- a/widget/nsGUIEvent.h
+++ b/widget/nsGUIEvent.h
@@ -1883,6 +1883,13 @@ public:
   }
 
   int32_t detail;
+
+  void AssignUIEventData(const nsUIEvent& aEvent, bool aCopyTargets)
+  {
+    AssignGUIEventData(aEvent, aCopyTargets);
+
+    // detail must have been initialized with the constructor.
+  }
 };
 
 class nsFocusEvent : public nsUIEvent
diff --git a/widget/tests/test_assign_event_data.html b/widget/tests/test_assign_event_data.html
index 93c6d82388be..b31b31b0f1ca 100644
--- a/widget/tests/test_assign_event_data.html
+++ b/widget/tests/test_assign_event_data.html
@@ -404,6 +404,17 @@ const kTests = [
     },
     todoMismatch: [ ],
   },
+  { description: "nsUIEvent (DOMActivate)",
+    targetID: "button", eventType: "DOMActivate",
+    dispatchEvent: function () {
+      synthesizeMouseAtCenter(document.getElementById(this.targetID),
+                              { button: 0, shiftKey: true });
+    },
+    canRun: function () {
+      return true;
+    },
+    todoMismatch: [],
+  },
 ];
 
 function doTest(aTest)