diff --git a/offlineimap/imapserver.py b/offlineimap/imapserver.py
index ef54e31..b7f062d 100644
--- a/offlineimap/imapserver.py
+++ b/offlineimap/imapserver.py
@@ -113,6 +113,7 @@ class IMAPServer:
 
         :param drop_conn: If True, the connection will be released and
            not be reused. This can be used to indicate broken connections."""
+        if connection is None: return #noop on bad connection
         self.connectionlock.acquire()
         self.assignedconnections.remove(connection)
         # Don't reuse broken connections