document.form undefined

[ANTAGONIA]

Buon giorno, vorrei sapere cortesemente come risolvere il problema di document.frmPopUp is undefined..

<script language="JavaScript" type="text/javascript">
function subPosNews() {
	if (!confirm("Modificare Posizione News?")) return;
	document.frmPopUp.action="?p=in_new_pos_news";
	document.frmPopUp.submit();
}
</script>

//form
<form name="frmPopUp" action="" method="post">

	<input type="text" name="txtPosNews" value="1"/>
	<input type="text" name="txtPosNews" value="2"/>
	<input type="text" name="txtPosNews" value="3"/>
	<input type="button" name="cmdConferma" style="width:auto !important" value="Conferma" onClick="subPosNews();"/>
</form>

Grazie a tutti......