[plt-scheme] An servlet exception difficult to comprehend

Hi,

While refactoring my web server application I encountered the
following error message:

play.ss:90:111: Servlet (@ /play) exception:
statement-name: expects args of type <struct:statement>; given
instance of a different <struct:statement>

The 'statement' is defined as a serializable struct:

 (define-serializable-struct statement (name type description))

What does the above error message want to say?