[plt-scheme] read (compiled): ill-formed code

I managed to get the following error from some code:

UNKNOWN: read (compiled): ill-formed code
[../../../mzscheme/gc2/../src/eval.c:11632]

 === context ===
/Users/noel/programming/bayesian-scan-matching/scan-match.ss:95:0:
scan-match/best
/Users/noel/programming/plt-trunk/collects/scheme/contract/private/arrow.ss:1437:3
/Users/noel/Library/PLT
Scheme/planet/300/4.2.5.1/cache/schematics/schemeunit.plt/3/4/result.ss:100:3
/Users/noel/Library/PLT
Scheme/planet/300/4.2.5.1/cache/schematics/schemeunit.plt/3/4/test-suite.ss:28:2
the-tests
/Users/noel/Library/PLT
Scheme/planet/300/4.2.5.1/cache/schematics/schemeunit.plt/3/4/test-suite.ss:60:0:
apply-test-suite
the-tests
/Users/noel/Library/PLT
Scheme/planet/300/4.2.5.1/cache/schematics/schemeunit.plt/3/4/test-suite.ss:60:0:
apply-test-suite
/Users/noel/Library/PLT
Scheme/planet/300/4.2.5.1/cache/schematics/schemeunit.plt/3/4/text-ui.ss:226:0:
run-tests
/Users/noel/Library/PLT
Scheme/planet/300/4.2.5.1/cache/schematics/sake.plt/1/0/actions.ss:9:0:
test
/Users/noel/programming/plt-trunk/collects/scheme/private/map.ss:45:11: for-each
/Users/noel/Library/PLT
Scheme/planet/300/4.2.5.1/cache/schematics/sake.plt/1/0/sake.ss:
[running body]


I've just been modifying some Typed Scheme code to use unsafe ops.
This is in 4.2.5.1. The C code in question is:

  if (typed_arg) {
    if ((proc_with_refs_ok != 1)
        && !argument_to_arity_error(app_rator, proc_with_refs_ok))
      scheme_ill_formed_code(port);
  }

I'll look for obvious errors. If anyone can explain the above code
that would help. I can put the code online if that's useful. The error
is reproducible.

Ta,
N.