Why invliad opcode always 0000

Every time seeing an invalid opcode, it always shows 0000, what does this 0000 mean? ex:

kernel BUG at arch/x86/kernel/cpu/mce/core.c:1364!
invalid opcode: 0000 [#1] SMP NOPTI

0000 means opcode is 0000, or execution result is 0000, or error code is 0000, or something else? btw, I can't find the kernel code where it prints this line, only: https://elixir.bootlin.com/linux/latest/source/lib/bug.c#L204

if (file)
		pr_crit("kernel BUG at %s:%u!\n", file, line);

not sure this place is the right place to print it.