diff --git a/smem b/smem
index 654c375..66e7a91 100755
--- a/smem
+++ b/smem
@@ -45,7 +45,7 @@ class procdata(object):
     def pidgroup(self, pid):
         return self._stat('%d/cmdline' % pid).st_gid
     def username(self, uid):
-        if uid not in _ucache:
+        if uid not in self._ucache:
             self._ucache[uid] = pwd.getpwuid(uid)[0]
         return self._ucache[uid]
     def groupname(self, gid):