diff --git a/pymemcache/test/utils.py b/pymemcache/test/utils.py
index 52b1732..3b05aab 100644
--- a/pymemcache/test/utils.py
+++ b/pymemcache/test/utils.py
@@ -102,6 +102,7 @@ def set(self, key, value, expire=0, noreply=True, flags=None):
 
         value, flags = self.serde.serialize(key, value)
 
+        assert isinstance(expire, int), expire
         if expire:
             expire += time.time()