Steps to reproduce:

1) emacs -Q
2) Evaluate the following form:

   (let ((l '("a" "b")))
     (completing-read-multiple "" l nil nil nil nil l))

3) C-m

I expect this to result in the string "a", as is the case when
completing-read-multiple is replaced with completing-read in step 2,
given the following excerpt from the docstring of c-r-m:

  The arguments are the same as those of `completing-read'.

Instead, the following error is signalled:

Debugger entered--Lisp error: (wrong-type-argument stringp ("a" "b"))
  string-match("[ \011]*,[ \011]*" ("a" "b") 0)
  split-string(("a" "b") "[ \011]*,[ \011]*" t)
  completing-read-multiple("" ("a" "b") nil nil nil nil ("a" "b"))

I attach a fix which follows the implementation of
completing-read-default.

Thanks,

-- 
Basil

In GNU Emacs 27.0.50 (build 14, x86_64-pc-linux-gnu, X toolkit, Xaw3d scroll bars)
 of 2018-01-10 built on thunk
Repository revision: b877e9aa0225ad186b544f127702e16c61b08444
Windowing system distributor 'The X.Org Foundation', version 11.0.11905000
System Description: Debian GNU/Linux testing (buster)