Troubleshooting error : "The conversation handle "XYZ" is not found"

We have a stored proc that generates service broker messages that are later processed / stored for long term via an activation stored proc.

Occasionally, we see the following error when the message sender sp is invoked:

TypeName: System.Data.SqlClient.SqlException, System.Data
Message: The conversation handle "A33AA310-379C-E611-BFAC-00154D09A706" is not found.
Source: .Net SqlClient Data Provider

Here is the message sender TSQL from the sender sp that generates the service broker message.  Not sure what would cause a conversation handle not found error here.

If (@MessageBody IS NOT NULL)
BEGIN
		SET @Handle = NEWID()
		BEGIN DIALOG CONVERSATION @Handle
		FROM SERVICE ImageAuditPublisher
		TO SERVICE 'ImageAuditSubscriber'
		ON CONTRACT [http://schemas.foo.com/contracts/ImageAudit]
		WITH ENCRYPTION = OFF;
		SEND ON CONVERSATION @Handle
		MESSAGE TYPE [http://schemas.foo.com/messages/ImageAudit/Insert](@MessageBody);
		END CONVERSATION @Handle

		print 'ImageAudit message sent!'
		print CONVERT(VARCHAR(MAX), @MessageBody, 1);
END

 What typically causes this type of error?

Microsoft SQL Server 2014 - 12.0.4213.0 (X64) 
Jun  9 2015 12:06:16 
Copyright (c) Microsoft Corporation
Standard Edition (64-bit) on Windows NT 6.1 <X64> (Build 7601: Service Pack 1) (Hypervisor)