Matlab Code Error(PSNR =inf)

I was runnning a code to calculate PSNR of compressed images .I was able to get PSNR values higher till 314 dB for compression ratio = 2 and below but the same program is now giving me PSNR=inf for compression ratio =8 and below and sometimes for CR =16 as well.

Please help .The code runs fine with no error but i am not able to get PSNR for low compression ratios and for PSNR above 90.

Regards