How to pass selected radio button to a parameter

I'm getting an error Object cannot be cast from dbnull.   All the Values for "is patient working" field in my database are Nulls. I want to use the radio buttons to update the NuLLs to No = 0 or Yes =1.   Thank you Amit and Vishai for your posts extremely helpful.

1. protected void rbl_SelectedIndexChanged(object sender, System.EventArgs e)  
2.     {  
3.         RadioButtonList rBtnList = (RadioButtonList)sender;     
4.             GridViewRow gvr = (GridViewRow)rBtnList.Parent.Parent;  
5.         if(rBtnList.SelectedValue == "1")  
6.             SqlDataSource1.UpdateParameters[0].DefaultValue = "True";  
7.         else  
8.             SqlDataSource1.UpdateParameters[0].DefaultValue = "False";  
9.         SqlDataSource1.UpdateParameters[1].DefaultValue = gvr.Cells[0].Text;  
10.         SqlDataSource1.Update();  
11.         GridView1.DataBind();         
12.     }