If called as `(self-insert-command)', you get
Wrong type argument: natnump, nil
While it's hard to see a use for that, maybe for robustness we should change the
CHECK_NATNUM (count = NILP (count) ? 1 : count);
since count is an optional argument. "count = Fprefix_numeric_value (count)" seems
wrong; the only non-integer argument that should be allowed is nil.