How do I get the relative error between 2 signals in Simulink?

I have two first order transfer functions in Simulink. One is the exact system and the other is a close approximation. How would I get the relative error between the two? I tried using an addition block to subtract the exact function from the approximation and then taking the output from the addition block and using a divide block to divide the absolute error by the output of the original transfer function to get the relative error. The addition block looks to be giving the correct absolute error, but then the divide block sends out a massive maximum relative error value, on the order of 10^23.