How to set Masterpage to site master?

Question

Hi,

I am opening the responsive site into mobile device.

It with switch master page to Site.Mobile.Master. So, code will not execute of Site.Master and return the null value error.

 

I need to set my included Site.Master when it is mobile device.

<%@ Page Title="Transaction" MasterPageFile="~/Site.Master" Language="C#" AutoEventWireup="true" CodeBehind="Trans.aspx.cs" Inherits="Merchant.Trans" %>