On the current master, I noticed this error when completing for some
function names, such as ``C-h f package-'':

Debugger entered--Lisp error: (wrong-type-argument stringp nil)
  string-match("\\(\\`\\|/\\)package-x\\(\\.elc\\|\\.el\\|\\.so\\)?\\(\\.gz\\)?\\'" nil nil)
  help--loaded-p("package-x")
  help--load-prefixes((("package-" "package-x" "package")))
  help--symbol-completion-table("package-" #[257 "\300!\206     \211\301N\207" [fboundp function-documentation] 3 "\n\n(fn F)"] lambda)
  test-completion("package-" help--symbol-completion-table #[257 "\300!\206     \211\301N\207" [fboundp function-documentation] 3 "\n\n(fn F)"])
  completion--do-completion(20 27)

It seems that I have a bare (nil) in the load-history, which
help--loaded-p doesn't like.

Is this expected?
This doesn't occur on emacs 25.1