The behavior is undefined if a value does not match its type declaration, and implementations are free to ignore some declarations, like CLISP. So for a portabl

The behavior is undefined if a value does not match its type declaration, and implementations are free to ignore some declarations, like CLISP. So for a portable approach just add a CHECK-TYPE: (defun foo (s) (check-type s string) (print s)) The main point is that (TYPEP NIL 'STRING) is NIL.