diff --git a/content/events/src/nsDOMEvent.cpp b/content/events/src/nsDOMEvent.cpp
index 622cb6a35ab9..4bb42baaece7 100644
--- a/content/events/src/nsDOMEvent.cpp
+++ b/content/events/src/nsDOMEvent.cpp
@@ -586,8 +586,7 @@ nsDOMEvent::DuplicatePrivateData()
         new nsCompositionEvent(false, msg, nullptr);
       nsCompositionEvent* oldCompositionEvent =
         static_cast<nsCompositionEvent*>(mEvent);
-      compositionEvent->AssignGUIEventData(*oldCompositionEvent, true);
-      compositionEvent->data = oldCompositionEvent->data;
+      compositionEvent->AssignCompositionEventData(*oldCompositionEvent, true);
       newEvent = compositionEvent;
       break;
     }
diff --git a/widget/nsGUIEvent.h b/widget/nsGUIEvent.h
index fc61cd8aeb29..e77ee97ca070 100644
--- a/widget/nsGUIEvent.h
+++ b/widget/nsGUIEvent.h
@@ -1395,6 +1395,14 @@ public:
   }
 
   nsString data;
+
+  void AssignCompositionEventData(const nsCompositionEvent& aEvent,
+                                  bool aCopyTargets)
+  {
+    AssignGUIEventData(aEvent, aCopyTargets);
+
+    data = aEvent.data;
+  }
 };
 
 /**
diff --git a/widget/tests/test_assign_event_data.html b/widget/tests/test_assign_event_data.html
index f79a18b3b160..f263c1646f74 100644
--- a/widget/tests/test_assign_event_data.html
+++ b/widget/tests/test_assign_event_data.html
@@ -241,6 +241,29 @@ const kTests = [
     },
     todoMismatch: [ ],
   },
+  { description: "nsCompositionEvent (compositionupdate)",
+    targetID: "input-text", eventType: "compositionupdate",
+    dispatchEvent: function () {
+      document.getElementById(this.targetID).value = "";
+      document.getElementById(this.targetID).focus();
+      synthesizeComposition({ type: "compositionstart" });
+      synthesizeComposition({ type: "compositionupdate", data: "\u30E9\u30FC\u30E1\u30F3" });
+      synthesizeText({ "composition":
+        { "string": "\u30E9\u30FC\u30E1\u30F3",
+          "clauses":
+          [
+            { "length": 0, "attr": 0 }
+          ]
+        },
+        "caret": { "start": 4, "length": 0 }
+      });
+      synthesizeComposition({ type: "compositionend", data: "\u30E9\u30FC\u30E1\u30F3" });
+    },
+    canRun: function () {
+      return true;
+    },
+    todoMismatch: [ ],
+  },
 ];
 
 function doTest(aTest)