I'm revising old Emacs bugs in Debian and will forward some of them that
still apply.  To reproduce this one:

emacs -Q

M-: (start-process "nop" (current-buffer) "sleep" "100")

In the *scratch* buffer, type "free", then with point after that word

M-x hippie-expand

Result is the error

Symbol's value as variable is void: comint-use-prompt-regexp

A possible solution is to (require 'comint) in hippie-exp.el.