Training loss goes to NaN

I started training with Celeb database, but around 50k iteration, the loss goes to NaN & k_t goes to 1. Have you seen this before. 10%|█████▎ | 49650/500000 [5:29:56<49:13:32, 2.54it/s][49650/500000] Loss_D: 0.112764 Loss_G: 0.053848 measure: 0.1190, k_t: 0.0458 10%|█████▎ | 49700/500000 [5:30:16<49:18:01, 2.54it/s][49700/500000] Loss_D: nan Loss_G: nan measure: nan, k_t: 1.0000

Having said that, the output at 49k is starting to look good. Below are my last good outputs (real, d_real, g, d_fake).

My training stats looks like this