panic with DEBUG_MEMGUARD on PowerPC

When tracking down a panic exposed by INVARIANTS, I tried setting  
DEBUG_MEMGUARD, so I could find the culprit that's trashing freed  
memory.  However, this causes a panic at bootup.  It shows up right  
after the first WARNING: WITNESS message, with the following:

panic: kmem_suballoc: bad status return of 3
cpuid = 0
KDB: stack backtrace:
0xd0004ad0: at kdb_backtrace+0x4c
0xd0004b40: at panic+0x224
0xd0004ba0: at kmem_suballoc+0x8c
0xd0004bd0: at kmeminit+0x1ac
0xd0004c20: at mi_startup+0x13c
0xd0004c50: at btext+0xc0

Tracing, and printf() debugging, I see arguments to  
vm_map_findspace(): start: 0xD0000000, length: 4246446080, and map- 
 >max_offset = 4026531839.

Beyond that, I'm lost with tracking this down.  Machine is a dual  
processor PowerPC G4, with 2GB RAM.

Anyone have any ideas?

Thanks,
Justin