breaking into ddb -> panic

using [alt] break to debugger I am getting the following panic while
some debugging output is scrolling on the console:

db> reset
panic: lock (sleep mutex) eventhandler not locked @ /usr/src/sys/kern/subr_eventhandler.c:272
cpuid = 1
time = 1314
KDB: stack backtrace:
db_trace_self() at db_trace_self
db_trace_self_wrapper() at db_trace_self_wrapper+0x38
vpanic() at vpanic+0x1a0
panic() at panic+0x48
witness_unlock() at witness_unlock+0x140
__mtx_unlock_flags() at __mtx_unlock_flags+0x54
eventhandler_find_list() at eventhandler_find_list+0xbc
kern_reboot() at kern_reboot+0x244
db_reset() at db_reset+0xec
db_command() at db_command+0x2f4
db_command_loop() at db_command_loop+0x58
db_trap() at db_trap+0x100
kdb_trap() at kdb_trap+0x350
handle_el1h_sync() at handle_el1h_sync+0x18
--- exception, esr 0xf2000000
kdb_alt_break_internal() at kdb_alt_break_internal+0x1a8
kdb_alt_break() at kdb_alt_break+0x10
uart_intr_rxready() at uart_intr_rxready+0x88
uart_intr() at uart_intr+0x124
intr_event_handle() at intr_event_handle+0xf4
intr_isrc_dispatch() at intr_isrc_dispatch+0x60
arm_gic_intr() at arm_gic_intr+0x118
intr_irq_handler() at intr_irq_handler+0x98
handle_el1h_irq() at handle_el1h_irq+0x18
--- interrupt
cpu_idle() at cpu_idle+0x78
sched_idletd() at sched_idletd+0x494
fork_exit() at fork_exit+0x78
fork_trampoline() at fork_trampoline+0x18
Uptime: 21m54s
Automatic reboot in 15 seconds - press a key on the console to abort


-- 
Bjoern A. Zeeb                                                     r15:7