--- a/layout/base/RestyleTracker.h
+++ b/layout/base/RestyleTracker.h
@@ -345,17 +345,19 @@ RestyleTracker::AddPendingRestyle(Elemen
       // the descendant.
       RestyleData* curData;
       mPendingRestyles.Get(cur, &curData);
 
       // Even if cur has a ForceDescendants restyle hint, we're not guaranteed
       // to reach aElement in the case the PresShell posts a restyle event from
       // PostRecreateFramesFor, so we need to track it here.
       MOZ_ASSERT(curData, "expected to find a RestyleData for cur");
-      curData->mDescendants.AppendElement(aElement);
+      if (curData) {
+        curData->mDescendants.AppendElement(aElement);
+      }
     }
   }
 
   mHaveLaterSiblingRestyles =
     mHaveLaterSiblingRestyles || (aRestyleHint & eRestyle_LaterSiblings) != 0;
   return hadRestyleLaterSiblings;
 }