[plt-scheme] Odd GL problem

I've been mucking about with generating GPU programs via scheme, 
and have hit an odd problem.

OpenGL defines a function "GetString" which returns assorted 
implementation data selected by an enum parameter. The fragment 
program GL extension adds an extra enum, program-error-string-arb,
to get compilation error messages out.  Using an extended sgl.ss, I 
have a glut-based application which embeds mzscheme; 
(get-string 'program-error-string-arb) works fine in that.  But, in 
a mred gl-context, it doesn't; it fails with "invalid enum".  The 
equivalent C code called in the same contexts has the same behaviour,
so I'm fairly sure it's not my extensions to sgl going wrong here.
I get the same behaviour on a locally built (debian unstable) version
of PLT 209, so I'm reasonably sure I'm linking against the right
libraries; (get-string 'vendor) gives the expected answer (nvidia, 
not mesa) in all cases.

Any thoughts?  I guessed that the wx stuff might be statically linking
mesa or something but grepping the sources doesn't seem to bear that
out.

John