diff --git a/src/calibre/gui2/cover_flow.py b/src/calibre/gui2/cover_flow.py
index b662f2e55f..b088d6da47 100644
--- a/src/calibre/gui2/cover_flow.py
+++ b/src/calibre/gui2/cover_flow.py
@@ -231,6 +231,7 @@ class CoverFlow(pictureflow.PictureFlow):
         return self.minimumSize()
 
     def wheelEvent(self, ev):
+        d = 0
         if ev.angleDelta().x():
             d = ev.angleDelta().x()
         if ev.angleDelta().y():