diff --git a/kivy/cache.py b/kivy/cache.py
index 48d9cd0166..18c71c2afe 100644
--- a/kivy/cache.py
+++ b/kivy/cache.py
@@ -193,7 +193,7 @@ def _purge_oldest(category, maxpurge=1):
         import heapq
         time = Clock.get_time()
         heap_list = []
-        for key in Cache._objects[category]:
+        for key in list(Cache._objects[category]):
             obj = Cache._objects[category][key]
             if obj['lastaccess'] == obj['timestamp'] == time:
                 continue