Error adding a monitoring server to a log shipping configuration

I am setting up log shipping for a database and have hit an error at the "monitor server" stage. I used the GUI to create the T-SQL that needs to be executed and it generated:

EXEC msdb.dbo.sp_processlogshippingmonitorsecondary 
	@mode = 1 
	,@secondary_server = N'SECONDARY' 
	,@secondary_database = N'DBNAME' 
	,@secondary_id = N'' 
	,@primary_server = N'PRIMARY' 
	,@primary_database = N'DBNAME' 
	,@restore_threshold = 45   
	,@threshold_alert = 14420 
	,@threshold_alert_enabled = 1 
	,@history_retention_period	= 5760 
	,@monitor_server = N'MONITOR' 
	,@monitor_server_security_mode = 1 

However when running this command I get the error:

Msg 8114, Level 16, State 5, Procedure sp_processlogshippingmonitorsecondary, Line 0 [Batch Start Line 198]
Error converting data type nvarchar to uniqueidentifier.

I'm guessing this is coming from the empty string associated with the @secondary_id parameter but I don't know whether this can be removed from the call to the stored procedure or needs to be filled with a value. If the latter, what value would it need to be?

Thanks.