Isotopic ratios in comets are critical to understanding the origin of cometary material and the physical and chemical conditions in the early solar nebula. Comet C/2015 ER61 (PANSTARRS) underwent an outburst with a total brightness increase of 2 magnitudes on the night of 2017 April 4. The sharp increase in brightness offered a rare opportunity to measure the isotopic ratios of the light elements in the coma of this comet. We obtained two high-resolution spectra of C/2015 ER61 with UVES/VLT on the nights of 2017 April 13 and 17. At the time of our observations, the comet was fading gradually following the outburst. We measured the nitrogen and carbon isotopic ratios from the CN violet (0, 0) band and found that C-12/C-13 = 100 +/- 15, N-14/N-15 = 130 +/- 15. In addition, we determined the N-14/N-15 ratio from four pairs of NH2 isotopolog lines and measured N-14/N-15 = 140 +/- 28. The measured isotopic ratios of C/2015 ER61 do not deviate significantly from those of other comets.