diff --git a/Lib/difflib.py b/Lib/difflib.py
index f2d1343..2dca749 100644
--- a/Lib/difflib.py
+++ b/Lib/difflib.py
@@ -331,7 +331,7 @@
         junkdict = {}
         if isjunk:
             for d in populardict, b2j:
-                for elt in d.keys():
+                for elt in list(d.keys()):
                     if isjunk(elt):
                         junkdict[elt] = 1
                         del d[elt]