--- a/gfx/layers/apz/src/APZCTreeManager.cpp
+++ b/gfx/layers/apz/src/APZCTreeManager.cpp
@@ -2548,16 +2548,22 @@ APZCTreeManager::SetLongTapEnabled(bool 
 {
   APZThreadUtils::AssertOnControllerThread();
   GestureEventListener::SetLongTapEnabled(aLongTapEnabled);
 }
 
 RefPtr<HitTestingTreeNode>
 APZCTreeManager::FindScrollThumbNode(const AsyncDragMetrics& aDragMetrics)
 {
+  if (!aDragMetrics.mDirection) {
+    // The AsyncDragMetrics has not been initialized yet - there will be
+    // no matching node, so don't bother searching the tree.
+    return RefPtr<HitTestingTreeNode>();
+  }
+
   RecursiveMutexAutoLock lock(mTreeLock);
 
   return DepthFirstSearch<ReverseIterator>(mRootNode.get(),
       [&aDragMetrics](HitTestingTreeNode* aNode) {
         return aNode->MatchesScrollDragMetrics(aDragMetrics);
       });
 }