Failed to enable constraints. One or more rows contain values violating non-null, unique, or foreign-key constraints.

ALTER PROCEDURE getEmailThemes
AS
BEGIN
	SELECT EpaperTheme.ID AS ThemeID, EpaperTheme.Title AS Title, EpaperTheme.CreationDate, MarketingCategory.Title AS Category
	FROM EpaperTheme JOIN MarketingCategory ON EpaperTheme.Category_ID = MarketingCategory.ID
END
GO

string ConnectionString = "Data Source=.;Initial Catalog=myDatabase;Integrated Security=SSPI";
            SqlConnection Connection = new SqlConnection(ConnectionString);
            string command = "SELECT * FROM MarketingCategory";
            SqlCommand Command = new SqlCommand(command, Connection);
            DataTable dt = new DataTable(); ;
            Connection.Open();
            SqlDataReader dr = Command.ExecuteReader();
            dt.Load(dr);
            NumberOfCategory = dt.Rows.Count - 1;
            //string[][] EmailArchives = new string[NumberOfCategory][];
            string[] Tbody = new string[NumberOfCategory];
            drpCategoryList.DataSource = dt;
            drpCategoryList.DataValueField = "Title";
            drpCategoryList.DataTextField = "Title";
            drpCategoryList.DataBind();

            dt.Clear();
            command = "getEmailThemes";
            Command.CommandText = command;
            SqlDataReader dre = Command.ExecuteReader();
            dt.Load(dre); //error

          

           Connection.Close();

Failed to enable constraints. One or more rows contain values violating non-null, unique, or foreign-key constraints.