valgrind and +heap not getting along?

[Dan Kegel]

I'm seeing fun traces like

==17177== Invalid write of size 1
==17177==    at 0x4022D5A: memset (mc_replace_strmem.c:492)
==17177==    by 0x46E914A: mark_block_free (heap.c:148)
==17177==    by 0x46E6950: HEAP_CreateFreeBlock (heap.c:493)
==17177==    by 0x46E6B78: HEAP_ShrinkBlock (heap.c:575)
==17177==    by 0x46E7F47: RtlAllocateHeap (heap.c:1228)
==17177==    by 0x46EC10A: get_builtin_fullname (winnt.h:1989)
==17177==    by 0x46EC2B8: load_builtin_callback (loader.c:1320)
==17177==    by 0x402C3BF: wine_dll_set_callback (loader.c:480)
==17177==    by 0x46EF3A7: __wine_process_init (loader.c:2529)
==17177==    by 0x402C7D5: wine_init (loader.c:645)
==17177==    by 0x7BF01263: main (main.c:111)
==17177==  Address 0x7f000140 is not stack'd, malloc'd or (recently) free'd

when running with +heap under valgrind.  Is something off by one somewhere?
- Dan