I cannot figure out what I did or did not do here syntactically to cause this error. I don't see what's missing:

function ShowWaitMessage(button)
{
    var isValid;

    if (buttonSelected())
    {
        showWaitMessage(button, "showMessage1");
        isValid = true;
    }
    else
    {
        Page_ClientValidate();
        if (Page_IsValid)
        {
            showWaitMessage(button, "showMessage2");
            isValid = true;
        }
    }

    return isValid;
}