[plt-scheme] Variable arity error?

Hi all,

I'm a bit confused by this error from v350.4 (or HEAD):

--
current-module-name-resolver: expects argument of type <procedure of 
arity 1, 3, and 4>; given #<procedure>

  === context ===
#f::353: loop
--

This is somewhere in a large body of code that was happy under v209. I'm 
downloading and building v300 to see if this same error is thrown there. 
  I assume this is one of those times that I hope that running my code 
with 'errortrace' gives me more information?

Cheers,
Matt