Lisp macros receive as input valid s-expressions, that don't need necessarily to be valid common lisp code, such as

    (my-macro (2 + 2))

and output valid lisp code during macro expansion time. The macro my-macro could expand the code to something like:

    (+ 2 2)

Some Lisp compilers such as CMUCL will issue a warning if you try to use wrong type (it has type inference), such as in this simple example:

    (defmacro foo ()
      `(+ 1 "foo"))

    (defun bar ()
      (foo))

You can see the compiler notes here: http://imgur.com/3vlQm6G