Hello,

More info is in the commit message.

To re-create the error open up an EPUB file (or possibly any non-PDF
file in doc-view) and press 'M-g i' to run 'imenu'.  Doing that I get
the following error:

Debugger entered--Lisp error: (wrong-type-argument stringp nil)
  string-match("\\`PK\\'" nil)
  imenu-find-default("PK" (("*Rescan*" . -99) nil))
  imenu--completion-buffer((("*Rescan*" . -99) nil) nil)
  imenu-choose-buffer-index()
  byte-code("\300 C\207" [imenu-choose-buffer-index] 1)
  command-execute(imenu)