Web Forms :: Don't Allow To Insert More Than 2 Items In Database

I have 2 table in data base

1-HOuse_info Table

2-House_p Table

in House_p table save users Product information and in House_info table save Users Information

In both Tables is column that name is BehcodeN

I want if users in BehcodeN save this text='free'  'they can't insert product morethan 2 number and if they want insert  their third product It show error that You can't insert morethan 2 product below are my code

In Product.aspx page I have button that when users click on it they can insert their product

  protected void ImageButton2_Click1(object sender, ImageClickEventArgs e)
{
string data = Server.UrlDecode(Request.QueryString["BehCode"]);
string price = RadioButton2.Checked ? TextBox1.Text : "null";
SqlCommand _cmd = new SqlCommand("insertproduct3", _cn);

[Code] ....

and SP

USE [behtop]
GO
/****** Object: StoredProcedure [dbo].[insertproduct3] Script Date: 03/14/2013 17:42:53 ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
ALTER procedure [dbo].[insertproduct3]

[Code] ....

ok here I can insert 2 product succesfully. but when I want insert third produt it makes below error instead of showing error message

Server Error in '/behtop website' Application.
Object cannot be cast from DBNull to other types.
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code. Exception Details: System.InvalidCastException: Object cannot be cast from DBNull to other types.Source Error: 

Line 805: _cmd.Parameters.AddWithValue("@behcode", data);
Line 806: _cmd.Parameters.AddWithValue("@id", Convert.ToInt32(ViewState["Id"]));
Line 807: int ID = Convert.ToInt32(_cmd.ExecuteScalar()); Line 808: if (ID > 0)
Line 809: {

why this happen?