Advice on dog sitting a dog that is shy and nipped me during meet&greet?

While sitting on the couch with the owner and the (small Maltese mix, 14 years old) discussing dog sitting information for about 30 minutes, I went to pet the dog and it bit me pretty hard in the hand. I had let the dog sniff me multiple times and warm up to me for 30 minutes, and once the dog laid down between me and the owner I assumed it was okay try to pet it. Right when I reached to pet it, it quickly turned and bit me hard. No warning growl or bearing of teeth for warning either. The bite didn't draw blood but there was definite redness and swelling of my hand that lasted for a day or so. The owner apologized profusely and said sometimes this happens when the dog isn't comfortable and gave me some treats to feed it to make "amends". The dog ate the treats from my hand fine, however I am still nervous for when I am house sitting without the owner present. I still agreed to dog sit because the owners seem desperate for the help and I am desperate for the money. I have been bit by two dogs before and would prefer to not make it three....

Any advice on how to prevent this from happening when I am dog sitting? and how to get on the dogs good side?