interactive argument not meeting condition

[Heime]

For the line column I test whether actm disables the value. 
What happens to line-column when the condition does not
invoke read-number?  Would I need some default for it?

(defun polaq (actm line-column)

  (interactive
    (let* ( (cseq '("extended" "disable" "tabtrail"))
            (actm (completing-read "Dragovis: " cseq nil t "tabtrail"))
            (line-column (if (not (string= actm "disable"))
                               (read-number "Set line column: " 72))) )

      (list actm line-column)) )