Here is my counter-example. It is the console output of Clisp 2.49: Break 1 [2]> (declaim (ftype (function (string) t) foo)) NIL Break 1 [2]> (defun foo (x)

Here is my counter-example. It is the console output of Clisp 2.49: Break 1 [2]> (declaim (ftype (function (string) t) foo)) NIL Break 1 [2]> (defun foo (x) (print x)) FOO Break 1 [2]> (foo "hello") "hello" "hello" Break 1 [2]> (foo nil) NIL NIL Break 1 [2]> There were no errors. The function accepted nil and printed it. You must have some other type restrictions going on than what you mentioned. The output is the same whether I put the declaim statement before or after defun foo.