Just run a vm, and right after, try to use bhyve with any option on the same vm name.
(gdb) run -Y fbsd1
Starting program: /usr/sbin/bhyve -Y fbsd1
[New LWP 100688]
Assertion failed: (error == 0), function fbsdrun_addcpu, file /usr/src/usr.sbin/bhyve/bhyverun.c, line 261.
[New Thread 801c06400 (LWP 100688/bhyve)]
Program received signal SIGABRT, Aborted.
[Switching to Thread 801c06400 (LWP 100688/bhyve)]
0x0000000800fd267a in thr_kill () from /lib/libc.so.7
(gdb) bt
#0 0x0000000800fd267a in thr_kill () from /lib/libc.so.7
#1 0x0000000800fd2628 in raise () from /lib/libc.so.7
#2 0x0000000800fd0e49 in abort () from /lib/libc.so.7
#3 0x0000000800fb1291 in __assert () from /lib/libc.so.7
#4 0x0000000000408559 in fbsdrun_addcpu (ctx=0x801c16080, fromcpu=0,
newcpu=0, rip=18446744071576357445)
at /usr/src/usr.sbin/bhyve/bhyverun.c:261
#5 0x000000000040903f in main (argc=1, argv=0x7fffffffeb38)
at /usr/src/usr.sbin/bhyve/bhyverun.c:884
(gdb) up 4
#4 0x0000000000408559 in fbsdrun_addcpu (ctx=0x801c16080, fromcpu=0,
newcpu=0, rip=18446744071576357445)
at /usr/src/usr.sbin/bhyve/bhyverun.c:261
261 assert(error == 0);
Current language: auto; currently minimal
(gdb) up 5
#5 0x000000000040903f in main (argc=1, argv=0x7fffffffeb38)
at /usr/src/usr.sbin/bhyve/bhyverun.c:884
884 fbsdrun_addcpu(ctx, BSP, BSP, rip);
(gdb) quit