At least secure it first and get a good track record down with the client. That way if you add anything to your business, you've already got a lot of credibility to help land the new business, or help some towards making sure a lapse in the one division isn't fatal to all the business. (In a nutshell, that's pretty much how I manage this particular risk).
My only other tip is if you do add another line one day, go all out to do it properly. Don't even try anything half-baked. Unfortunately the world tends to score more heavily on our weaknesses than our strengths, and a weak divsion can really drag down a strong one.
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