BEGIN DIALOG CONVERSATION returns NULL

Question

I experienced an issue with the SQL Server service broker. If I quickly run several times a the script below.

BEGIN DIALOG CONVERSATION @DBHandle
FROM SERVICE A
TO SERVICE N'B'
ON CONTRACT CommandExec
WITH ENCRYPTION = OFF

Sometimes @DBHandle has value null. I noticed it because the SEND ON CONVERSATION fails with "The conversation handle is missing. Specify a conversation handle".

Are there any structural limits on SQL Server? I expected an error, why a null value is returned?

Thanks, Dan