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I'm having some problem writing a LISP function. The function is defined as 

(defun foo (arg1 &optional cont)) (cond ((null arg1) nil) ((= 0 cont) arg1) ((do_something)) ((recursive call))))

When i call the function with cont everything works fine, but when I call it just with arg1 the error returned is: 

Error: in ZEROP of (NIL) arguments should be of type NUMBER

I guess something is wrong in the condition ((= 0 cont) arg1), can you help me solve this problem? Thanks

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    If you want to know if an optional argument is provided, you can do that: (defun foo (arg1 &optional (cont nil contp)) ... for instance Commented Feb 1, 2017 at 17:38
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    Btw., the error message seems to indicate that the call to = has been replaced by a call to zerop by the compiler. Commented Feb 1, 2017 at 19:56

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The = function, along with some other ones, expect exclusively numbers.

You need to use EQL or a more general equality comparison (equal, equalp) when you expect NIL to be a valid input; here, NIL is expected because it is the default value of the optional argument.

You could also provide a numerical default value to cont:

 ... &optional (cont 0) ...

... which might be the correct approach if cont has no reason to be anything else than a number.

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Alternative: the condition could be (and (numberp cont) (= 0 cont)) if cont might have a reason not to be a number.