If Else Statement not working ASP.NET

Index.aspx

 <asp:RadioButton ID="rbt1" runat="server" Text="By Customer" GroupName="summary" OnCheckedChanged="enabled_CheckedChanged" Checked="True"/>
 <asp:RadioButton ID="rbt4" runat="server" Text="With Target" OnCheckedChanged="enabled_CheckedChanged" GroupName="summary"/>

Index.aspx.cs

public void enabled_CheckedChanged(object sender, EventArgs e)
    {
       if (rbt4.Checked == true)
            {
            rbt1.Checked = true;
        }
       else
        {
            rbt1.Checked = false;
        }
    }

The purpose of the code is when rbt1 is checked, rbt4 will be be checked. User can also check rbt 4 after checking rbt 1. However, if rbt 4 is checked, rbt 1 will be checked automatically. The code above was used but it doesn't seem to be working. Did I miss out something or is there error in my coding? Please advice. Thanks in advance.