zHelp crashes when F1 is pressed twice

void zHelp::enableHelpKey(int enable) {
    if (enable) {
        if (!helpKeyEnabled) {
            zHelp::_ziHelpKeyHelpObj=this;
            lpfnFilterProc = (HOOKPROC) MakeProcInstance(
                
(FARPROC)helpKeyFilterFunction,zAppGetAppVar(app)->instance());
#ifndef __NT__
            hook = SetWindowsHookEx(WH_KEYBOARD,
                lpfnFilterProc,zAppGetAppVar(app)->instance(),
                    (HTASK)GetCurrentTask());
#else
            DWORD CurThreadId = GetCurrentThreadId();
            hook = SetWindowsHookEx(WH_KEYBOARD,
                lpfnFilterProc,NULL,CurThreadId);
#endif
            helpKeyEnabled = 1;
        }
    } else {
        if (helpKeyEnabled) {
            if (zHelp::_ziHelpKeyHelpObj==this)
zHelp::_ziHelpKeyHelpObj=0;
            UnhookWindowsHookEx(hook);
            FreeProcInstance((FARPROC)lpfnFilterProc);
            helpKeyEnabled = 0;
        }
    }
}