diff --git a/bin/tools/misc.py b/bin/tools/misc.py
index a2712ee006e..9d4ea4d66cd 100644
--- a/bin/tools/misc.py
+++ b/bin/tools/misc.py
@@ -659,9 +659,9 @@ class cache(object):
             if time.time()-self.timeout > self.lasttime:
                 self.lasttime = time.time()
                 t = time.time()-self.timeout 
-                for key in self.cache.keys():
-                    if self.cache[key][1]<t:
-                        del self.cache[key]
+                old_keys = [key for key in self.cache if self.cache[key][1] < t]
+                for key in old_keys:
+                    del self.cache[key]
 
             kwargs2 = self._unify_args(*args, **kwargs)