diff --git a/lib/rack/mock.rb b/lib/rack/mock.rb
index b9899efd7..d9c8a9cef 100644
--- a/lib/rack/mock.rb
+++ b/lib/rack/mock.rb
@@ -188,7 +188,7 @@ def initialize(status, headers, body, errors = nil)
       if errors
         @errors = errors.string if errors.respond_to?(:string)
       else
-        @errors = nil
+        @errors = ""
       end
 
       super(body, status, headers)