Typing "C-h k x" to try to find out what "x" is bound to fails:

Debugger entered--Lisp error: (wrong-type-argument stringp t)
  describe-function-1(self-insert-command)
  describe-key("x" 1 nil)
  call-interactively(describe-key)

The same happens whichever key sequence I try to describe.

I've tried a "make bootstrap" but the problem persists.