diff --git a/synapse/handlers/federation.py b/synapse/handlers/federation.py
index 43d1ec859d..be2ea0cd56 100644
--- a/synapse/handlers/federation.py
+++ b/synapse/handlers/federation.py
@@ -174,11 +174,9 @@ class FederationHandler(BaseHandler):
 
                         # Update the set of things we've seen after trying to
                         # fetch the missing stuff
-                        have_seen = yield self.store.have_events(
-                            [ev for ev, _ in pdu.prev_events]
-                        )
+                        have_seen = yield self.store.have_events(prevs)
 
-                        seen = set(have_seen.keys())
+                        seen = set(have_seen.iterkeys())
                         if prevs - seen:
                             logger.info(
                                 "Still missing %d prev events for %s: %r...",
@@ -231,19 +229,15 @@ class FederationHandler(BaseHandler):
         Args:
             origin (str): Origin of the pdu. Will be called to get the missing events
             pdu: received pdu
-            prevs (str[]): List of event ids which we are missing
+            prevs (set(str)): List of event ids which we are missing
             min_depth (int): Minimum depth of events to return.
-
-        Returns:
-            Deferred<dict(str, str?)>: updated have_seen dictionary
         """
         # We recalculate seen, since it may have changed.
         have_seen = yield self.store.have_events(prevs)
         seen = set(have_seen.keys())
 
         if not prevs - seen:
-            # nothing left to do
-            defer.returnValue(have_seen)
+            return
 
         latest = yield self.store.get_latest_event_ids_in_room(
             pdu.room_id