diff --git a/Lib/linecache.py b/Lib/linecache.py
index 89383ea..0501a10 100644
--- a/Lib/linecache.py
+++ b/Lib/linecache.py
@@ -45,7 +45,7 @@
     (This is not checked upon each call!)"""
 
     if filename is None:
-        filenames = cache.keys()
+        filenames = list(cache.keys())
     else:
         if filename in cache:
             filenames = [filename]