Failed To Convert Parameter Value From A String To A Guid Error

Hi, i have some problems passing a guid parameter to a stored procedure; the code is below and the error i get is;

 Failed to convert parameter value from a String to a Guid

  conn = new SqlConnection(ConfigurationManager.ConnectionStrings["LocalSqlServer"].ConnectionString);
cmd = new SqlCommand("spInsKeyswap_history", conn);
cmd.CommandType = CommandType.StoredProcedure;
conn.Open();

MembershipUser mu = Membership.GetUser(User.Identity.Name);
guid gdUserID = mu.ProviderUserKey;

cmd.Parameters.Add("@user_title", SqlDbType.VarChar, 50);
cmd.Parameters.Add("@first_name", SqlDbType.VarChar, 20);
cmd.Parameters.Add("@last_name", SqlDbType.VarChar, 50);
cmd.Parameters.Add("@email", SqlDbType.VarChar, 50);
cmd.Parameters.Add("@birthday", SqlDbType.VarChar, 15);
cmd.Parameters.Add("@alternate_number", SqlDbType.VarChar, 50);
cmd.Parameters.Add("@msisdn", SqlDbType.VarChar, 50);
cmd.Parameters.Add("@call1", SqlDbType.VarChar, 50);
cmd.Parameters.Add("@call2", SqlDbType.VarChar, 50);
cmd.Parameters.Add("@call3", SqlDbType.VarChar, 50);
cmd.Parameters.Add("@new_sim_msidn", SqlDbType.VarChar, 50);
cmd.Parameters.Add("@dealer_id", SqlDbType.UniqueIdentifier);
cmd.Parameters.Add("@status_code", SqlDbType.Int);
cmd.Parameters.Add("@support_id", SqlDbType.Int);
cmd.Parameters.Add("@return_value", SqlDbType.Int);

cmd.Parameters["@user_title"].Value = txtTitle.Text.ToString();
cmd.Parameters["@first_name"].Value = txtFirstName.Text.ToString();
cmd.Parameters["@last_name"].Value = txtLastName.Text.ToString();
cmd.Parameters["@email"].Value = txtEmailAddress.Text.ToString();
cmd.Parameters["@birthday"].Value = txtBirthday.Text;
cmd.Parameters["@alternate_number"].Value = txtAlternate.Text.ToString();
cmd.Parameters["@msisdn"].Value = txtNew.Text.ToString();
cmd.Parameters["@call1"].Value = txtCall1.Text.ToString();
cmd.Parameters["@call2"].Value = txtCall2.Text.ToString();
cmd.Parameters["@call3"].Value = txtCall3.Text.ToString();
cmd.Parameters["@new_sim_msidn"].Value = txtOld.Text.ToString();
//get logged in users user_id from membership
cmd.Parameters["@dealer_id"].Value = gdUserID;
cmd.Parameters["@status_code"].Value = 1;
cmd.Parameters["@support_id"].Value = 0;


cmd.Parameters["@return_value"].Direction = ParameterDirection.ReturnValue;
cmd.ExecuteNonQuery();

int returnValue = (int)cmd.Parameters["@return_value"].Value;