diff --git a/oml/cache.py b/oml/cache.py
index 7c5368d..22b0471 100644
--- a/oml/cache.py
+++ b/oml/cache.py
@@ -8,7 +8,7 @@ class Cache(dict):
 
     def get(self, key):
         now = time.time()
-        for key, value in self._added.items():
+        for key, value in list(self._added.items()):
             if value < now:
                 del self._added[key]
                 del self[key]