Basic Numerical Skills - Question and answers

I've been manually entering data for my monthly expenses, and when I try to compute the percentage change from one period to another, I frequently get errors or inconsistent results. For instance, if last month's expense was $0 or negative, my standard formula breaks. I've tried using =(B2-A2)/A2*100 in Excel, but it doesn't handle edge cases well. How can I adjust this to be more robust and accurate?