Nested Database Transactions In Forms

This should be a fairly simple question. It's based on this error message:
"Transaction count after EXECUTE
indicates that a COMMIT or ROLLBACK TRANSACTION statement is missing.
Previous count = 1, current count = 0."
I get this when executing a stored procedure upon processing a form. This error happens when I intentionally provide input to the stored procedure that I know should cause it to error out. I catch the exception, and it contains the error message, but it also contains the above message added on to it, which I don't want.
 
I won't post the entire stored procedure. But I'll list a digest of it (Just those lines that are significant). Assume that what's included is what happens when I provide bad input:
BEGIN
BEGIN TRY
BEGIN TRANSACTION
RAISERROR('The item selected does not exist in the database.', 16, 1);
COMMIT -- This won't execute when the RAISERROR breaks out to the CATCH block
END TRY 
BEGIN CATCH
ROLLBACK
DECLARE  @ErrorSeverity INT,   @ErrorMessage NVARCHAR(4000)
SET @ErrorSeverity = ERROR_SEVERITY()
SET @ErrorMessage = ERROR_MESSAGE()
RAISERROR(@ErrorMessage, @ErrorSeverity, 1)
END CATCH 
END 
 
 Okay, so that works fine. The problem is when I execute this with an SqlCommand object, on which I've opened a transaction. I won't include the entire setup of the data (with the parameters, since those seem fine), but I'll give my code that opens the connection and executes the query:
 
        con.Open();
        SqlTransaction transaction = con.BeginTransaction();
        command.Transaction = transaction;

        try
        {
            command.ExecuteNonQuery();
            transaction.Commit();
        }
        catch (Exception ex)
        {
            transaction.Rollback();
        }
        finally
        {
            con.Close();
        }
 
I'm calling the stored procedure listed above (which has its own transaction), using a SqlCommand object on which I've opened a transaction. When there is no error it works fine. But when I give the stored procedure bad data, it gives me that message about the transaction count.
Is there something I need to do in either my SQL or my C# to handle this? The entire message found in the Exception's Message is a concatenation of the message in my RAISERROR, along with the transaction count message I quoted at the beginning.
 
Thanks, 
-Dan