Button "is not a member of 'ASP."

I have added a button to my form which is calling some javascript to open it up as a POPup... I keep getting this error:
'btnFindPO_Click' is not a member of 'ASP.softwarelist_aspx'.
What could be causing this when on a similar for this approach works well? Could I just do it in VB.net instead of Java and still control the POPup in the same way?
<script type="text/javascript">
        function OpenPopup() {
            window.open("ViewPOs.aspx", "List", "scrollbars=yes,resizable=no,width=700,height=600");
            return false;
        }
</script>
<asp:Button ID="btnFindPO" runat="server" Height="23px"
                     Text ...