Access violation in gcx.Gcx.mark

[bobef]

Unhandled Exception: EXCEPTION_ACCESS_VIOLATION(0xc0000005) at gcx.Gcx.mark 
(0x00464194) thread(1652)
->us
#0 void gcx.Gcx.mark(void*, void*, void*) () from gcx

Any idea what this is? I'm using some revision of Tango between 0.99.7 and 
0.99.8 with dmd 1.035.

Thanks!