diff --git a/lib/rack/test.rb b/lib/rack/test.rb
index 6b010598..284d4d1d 100644
--- a/lib/rack/test.rb
+++ b/lib/rack/test.rb
@@ -36,6 +36,7 @@ class Session
       def initialize(mock_session)
         @headers = {}
         @env = {}
+        @digest_username = nil
 
         if mock_session.is_a?(MockSession)
           @rack_mock_session = mock_session