The best overloaded method match for 'SampleDataContext.passwrdchange(long?, string, string, ref char?)' has some invalid arguments

protected void BtnSave_Click(object sender, EventArgs e)
   {
       using (SampleDataContext dbContext = new SampleDataContext())
       {
           System.Nullable<char> outpt = null;
           long id = Convert.ToInt64(Session["USERID"]);
           dbContext.passwrdchange(id, old.Text.ToString().Trim(), newpass.Text.ToString().Trim(), ref outpt);

           if(outpt.ToString()=="T")
           {
               ClientScript.RegisterStartupScript(this.GetType(), "SuccessMsg", "<script>alert('Password Changed Successfully')</script>");
           }

           else
           {
               ClientScript.RegisterStartupScript(this.GetType(), "ErrorMsg", "<script>alert('Old Password Does Not Match with Records.')</script>");
           }
       }
   }

private void UpdateUser_Information(User_Information obj)
	{
		this.passwrdchange(((System.Nullable<long>)(obj.ID)), default(string), default(string),default(System.Nullable<char>));
	}

The best overloaded method match for 'SampleDataContext.passwrdchange(long?, string, string, ref char?)' has some invalid arguments

Argument 4 must be passed with the 'ref' keyword