loss becomes nan about classical-piano-composer

[GeorgeShimakawa]

Pardon for my ambiguous question.
I changed backend to an AMD GPU "PlaidML" by adding a line (os.environ["KERAS_BACKEND"] = "plaidml.keras.backend"). Platform is iMac Late2015, OSX-Mojave, Python 3.6.5
After parsing about 60 midi files, first loss was indicated as 'nan' as follows:
.
.
.
Parsing midi_songs/sonat-5.mid
Parsing midi_songs/352.mid
Parsing midi_songs/384.mid
Parsing midi_songs/5112.mid
Parsing midi_songs/sonat-4.mid
INFO:plaidml:Opening device "metal_amd_radeon_r9_m390.0"
Epoch 1/200
INFO:plaidml:Analyzing Ops: 2104 of 29742 operations complete
INFO:plaidml:Analyzing Ops: 5776 of 29742 operations complete
INFO:plaidml:Analyzing Ops: 9603 of 29742 operations complete
INFO:plaidml:Analyzing Ops: 14197 of 29742 operations complete
INFO:plaidml:Analyzing Ops: 19025 of 29742 operations complete
INFO:plaidml:Analyzing Ops: 24197 of 29742 operations complete
INFO:plaidml:Analyzing Ops: 27148 of 29742 operations complete
720/43019 [..............................] - ETA: 1:39:10 - loss: nan

Even when I continued further the the loss continues to be nan.

[GeorgeShimakawa]

OpenCL version is 1.2 and the gpu is AMD Radeon R9 M390. CPU is Intel Core i5.
PlaidML is 0.3.5. Memory is 16GB.