Use a math-based PEG ratio, not the one popularly used which is garbage and only happens to appear useful for a small range of values.
Please investigate the math based PEG ratio developed by Thomas Blankenhorn: P/(12E(1+G)^2). Here the 12 denotes what is commonly referred to as a fair market PE value. It just so happens that the popularly used PEG ratio formula works is because the growth rate is sometimes around 20%=0.20 and thus 12*(1+0.20)^2=17.3 ~ 20, but values away from this would thus be increasingly erroneous. The presently used formula does not use the growth rate as a percentage but a hundred times the percentage and just luckily appears to make sense for very conforming numbers but there is no mathematical foundation. Had there been a math foundation, the growth rate of zero and negative growth rates would not cause the model to foul up. You really should use a PEG ratio formula that does what it should.