panic: uma_zalloc_arg: Returning an empty bucket.

I just got this panic on my 11.0-CURRENT machine while building ports:

panic: uma_zalloc_arg: Returning an empty bucket.
cpuid = 0
KDB: stack backtrace:
db_trace_self_wrapper(c113330c,2,10000000,ee3fd850,ee3fd848,...) at db_trace_self_wrapper+0x2d/frame 0xee3fd810
kdb_backtrace(c12f131f,0,c116b406,ee3fd8e4,c116b406,...) at kdb_backtrace+0x30/frame 0xee3fd878
vpanic(c1410ab8,100,c116b406,ee3fd8e4,ee3fd8e4,...) at vpanic+0x11f/frame 0xee3fd8b4
kassert_panic(c116b406,0,c116b3eb,883,1,...) at kassert_panic+0xea/frame 0xee3fd8d8
uma_zalloc_arg(c26d58c0,0,101,0,0,...) at uma_zalloc_arg+0x4bd/frame 0xee3fd918
vm_radix_insert(d25b6780,c403c194,c116d974,3c7,716,...) at vm_radix_insert+0xfa/frame 0xee3fd960
vm_page_insert_after(6348,0,c403c150,637,0,...) at vm_page_insert_after+0xc1/frame 0xee3fd98c
vm_page_alloc(d25b6750,6348,0,40,ee3fdacc,...) at vm_page_alloc+0x4fe/frame 0xee3fd9b8
vm_fault_hold(c99b7000,8e9f1000,2,0,0,...) at vm_fault_hold+0x3cb/frame 0xee3fdb18
vm_fault(c99b7000,8e9f1000,2,0,c99b7000,...) at vm_fault+0x82/frame 0xee3fdb40
trap_pfault(8e9f1000,ee3fdd08,0,c9252620,c141e780,...) at trap_pfault+0x198/frame 0xee3fdbb8
trap(ee3fdd08) at trap+0x5e2/frame 0xee3fdcfc
calltrap() at calltrap+0x6/frame 0xee3fdcfc
--- trap 0xc, eip = 0x88607643, esp = 0xbfbfce98, ebp = 0xbfbfced8 ---
KDB: enter: panic


FreeBSD scratch.catspoiler.org 11.0-CURRENT FreeBSD 11.0-CURRENT #60 r258423M: Thu Nov 21 02:27:29 PST 2013     dl_at_scratch.catspoiler.org:/usr/obj/usr/src/sys/GENERICSMB  i386