unreadable forms as default in macros

[sshteingold@cctrading.com]

     I am getting an error when trying to compile or evaluate a function 
     that calls a macro with an optional argument that defaults to a value 
     with an unreadable printed representation. 
     
     E.g..
     > (defmacro zz (&optional (rt *readtable*)) rt)
     
     zz
     > (defun xx () (zz))
     
     xx
     > (xx)
     
     *** - EVAL: illegal form #<readtable #x000003F0>
     1. Break> abort
     
     > (compile 'xx)
     
     ERROR in function xx :
     Invalid form: #<readtable #x000003F0>
     nil
     > 
     
     So, what do I do wrong?