Need help about addeventlistener in conditional

Code:

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while (num<24) {
        MovieClip("key" + num + "_mc").addEventListener(MouseEvent.CLICK, keyboardUp);
        num++;
}

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so simple but I get this error:

Code:

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TypeError: Error #1034: Tür Dayatması başarısız oldu: "key1_mc", flash.display.MovieClip öğesine dönüştürülemiyor.
        at Untitled_fla::MainTimeline/frame1()

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PHP Code:

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while (num<24) {
    this.getChildByName("key" + num + "_mc").addEventListener(MouseEvent.CLICK, keyboardUp);
    num++;
} 


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Thanks for the reply. That works fine now.