Using SqlCommand.InsertParameter.Add to add a numeric field to a database from a webpage.

Hello Experts -

I am trying to create an invoice number that automatically increments each time an account is created.  I can get it to return the value fine, but everytime it tries to execute the line of code where it stores this value back into the DB for that users account I am getting an error.  Here is my code:

This creates the InvoiceNum, it works fine:


    private double InvoiceNum()
    {

        SqlDataSource DataSource = (SqlDataSource)CreateUserWizard1.CreateUserStep.ContentTemplateContainer.FindControl("InsertExtraInfo");
        string conString = DataSource.ConnectionString;
        SqlConnection cwConn = new SqlConnection(conString);
        cwConn.Open();
        MembershipUser User = Membership.GetUser(CreateUserWizard1.UserName);
        object UserGUID = User.ProviderUserKey;
        string myQuery = "SELECT Top 1 [InvoiceNum] from UserInfo ORDER BY [InvoiceNum] DESC;";
        SqlCommand invNumCommand = new SqlCommand(myQuery, cwConn);
        invNumCommand.CommandType = CommandType.Text;
        object objInvNum = invNumCommand.ExecuteScalar();
        cwConn.Close();
        cwConn.Dispose();
        double invNum = System.Convert.ToDouble(objInvNum);
        return  invNum + 1;
    }

This is my code to update the user account and enter the info into the UserInfo table:


    public void ExtraInfo(bool flagCCGood)
    {
        SqlDataSource DataSource = (SqlDataSource)CreateUserWizard1.CreateUserStep.ContentTemplateContainer.FindControl("InsertExtraInfo");
        MembershipUser User = Membership.GetUser(CreateUserWizard1.UserName);
        object UserGUID = User.ProviderUserKey;
        DataSource.InsertParameters.Add("UserId", UserGUID.ToString());
        DataSource.InsertParameters.Add("DateRegistered", DateTime.Now.ToString());
        DataSource.InsertParameters.Add("IPAddress", Request.UserHostAddress.ToString());
        DataSource.InsertParameters.Add("ActiveUser", flagCCGood == true ? "1" : "0" );
        string invoiceNumChk = Convert.ToString(InvoiceNum());  // works fine, returns the value of InvoiceNum
        DataSource.InsertParameters.Add("InvoiceNum", invoiceNumChk); // still shows the value
        DataSource.Insert(); // crashes here.  giving the following error:
}

 Cannot insert the value NULL into column 'InvoiceNum', table 'master.dbo.UserInfo'; column does not allow nulls. INSERT fails.
The statement has been terminated.

I am clearly getting a value for this field and have converted to a string like the insertparameter wants.  But somehow or another I am losing it once it goes to update?  the datatype in the SQL DB is a nchar(10) that does not accept nulls.

Thanks for any help!!