Problems with uffi and ACL5.0.1

Hi,
I am trying to load Uffi on a professional version of allegro v.5.0.1 but 
when I run (asdf:operate 'asdf:load-op :uffi) I get the following error:

Error: keyword list (:RETURNING :UNSIGNED-INT :CALL-DIRECT T 
:STRINGS-CONVERT NIL) should only contain keys (:OPTIMIZE-FOR-SPACE 
:ARG-CHECKING :RELEASE-HEAP :METHOD-INDEX :RETURNING :CONVENTION :CALLBACK 
:CALL-DIRECT)
[condition type: PROGRAM-ERROR]

However everything works under allegro v6.2 trial.

Can anyone tell me or give me a hint of what should I do to fix this error?

Thanks

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