ASP.NET MVC An exception of type 'System.Data.SqlClient.SqlException' occurred in System.Data.dll but was not handled in user code

Question

I'm using ASP.NET MVC 4 and I'm trying to save data to the database and this error always pops up:

My code:

// Save use to the database
public int CreateUser(UsersModel entity)
{
    using (SqlConnection conn = new SqlConnection(DBCon))
    {
        conn.Open();

        SqlCommand cmd = new SqlCommand("SaveUser", conn);   // call stored procedure
        cmd.CommandType = System.Data.CommandType.StoredProcedure;

        cmd.Parameters.AddWithValue("@Firstname", entity.Firstname);
        cmd.Parameters.AddWithValue("@Middlename", entity.Middlename);
        cmd.Parameters.AddWithValue("@Surname", entity.Surname);
        cmd.Parameters.AddWithValue("@Email", entity.Email);

        int rs = cmd.ExecuteNonQuery();

        return rs;
    }
}