The session keys for this conversation could not be created or accessed. The database master key is required for this operation. Error

Hi,

I'm getting the following error In sys.transmission_queue in the transmission_status column "The session keys for this conversation could not be created or accessed. The database master key"

I'm using only one Server to Send Messages but I thought you only need the master key if there are two or more Servers used in the Messaging?

Thanks in advance