The following works for me:
select 'job_ID: ' + cast(job_ID as varchar(100)) from sysjobs
Also the following works:
declare @job_id uniqueidentifier
select top 1 @job_id = job_id from sysjobs
print 'job_ID: ' + cast(@job_ID as varchar(100))
Is it possible your variable @job_id is NULL? ie. you haven't fetched a value into it?