CLIPS - How can I exclude characters from IF/READ statement's exceptions?

The code is asking a y/n question to the user and making a change - simple. The statement seems to accept only integer and float types and I need only two answers, so I used 1 and 0, and excluded the rest, BUT it reads only numbers, so only the numbers are excluded, not characters.

(defrule rule01
    =>
    (printout t "Question (yes=1/no=0)?" crlf)
    (bind ?x (read))
    (if (!= ?x 1)
        then
        (if (= ?x 0)
            then
            (assert (rule01 no))
        else (printout t "Use ONLY 0 OR 1 for your answers!" crlf))
    else (assert (rule01 yes))))

Currently, when you try to type in a character, it returns the following:

CLIPS> (run)
Question (yes=1/no=0)?
g
[ARGACCES5] Function <> expected argument #1 to be of type integer or float
[PRCCODE4] Execution halted during the actions of defrule rule01.

How can I put in an exception for characters?