Unable to access a text field (text box)

Hello,

Following is my code block:

<asp:TemplateField HeaderText="Pay Order No.">
                         <ItemTemplate>
                             <asp:TextBox ID="txtPay" runat="server" 
                                 Width="100px"></asp:TextBox>
                             </ItemTemplate>
                     </asp:TemplateField>

My Javascript Function:
function VerifyPayOrder() {
var payAmount = document.getElementById('<%=txtPay.ClientID%>').val; // I GET ERROR HERE AS txtPay can not be found on compile of the form...
}
return Ok;
}

Please help me to solve the ERROR...

Best Regards,
Rohan Laghate