Well, something must be spawning a thread first. From the Obj-C source¹, the comment on performForkChildInitialize() (the point where this crash happens) says
> Exception: processes that are single-threaded when fork() is called have no restrictions on +initialize in the child. Examples: sshd and httpd.
> Exception: processes that are single-threaded when fork() is called have no restrictions on +initialize in the child. Examples: sshd and httpd.
¹ https://opensource.apple.com/source/objc4/objc4-723/runtime/...