diff --git a/zipline/assets/assets.py b/zipline/assets/assets.py
index 9e0489e7..3124cab0 100644
--- a/zipline/assets/assets.py
+++ b/zipline/assets/assets.py
@@ -667,7 +667,11 @@ class AssetFinder(object):
         return [contracts[sid] for sid in sids]
 
     def lookup_expired_futures(self, start, end):
+        if not isinstance(start, pd.Timestamp):
+            start = pd.Timestamp(start)
         start = start.value
+        if not isinstance(end, pd.Timestamp):
+            end = pd.Timestamp(end)
         end = end.value
 
         fc_cols = self.futures_contracts.c