Mathcad does not calculate regions until either you scroll the worksheet until they are visible, or you hit Ctrl F9. The only thing you can really do about it is to trap the error and handle it more gracefully. If you click on the button in the attached worksheet when it's first opened it tells you the string has not been calculated. If you either scroll down to see the assignment or hit Ctrl F9, then click on the button, it gets the value.
Richard