testcover-start fails with the message "Wrong type argument: listp, \,"
when asked to instrument lisp/emacs-lisp/cconv.el from the Emacs source
tree. I've simplified the problem down to this fragment of code:

(pcase form
    (`(condition-case ,var ,protected-form . ,handlers)
     nil))

To reproduce, create a file called bug.el containing the code above and:
1. emacs -Q
2. M-x toggle-debug-on-error RET
3. C-x C-f bug.el RET
4. M-: (require 'testcover) RET
5. M-x testcover-start RET bug.el RET

Here is the resulting backtrace: