Сharacters "?" in translation

I translate English into Russian with OpenNMT-tf. And I constantly meet in the translation symbols “??”. And the problem occurs both at 5 000 and 50 000 thousand steps (45 - 67 bleu).
I use a dictionary of 15000 words.

Hi Artem,

The dataset is very small. You can download more data from OPUS.

If you want to run a test with this small dataset, you can decrease the learning rate, but this doesn’t guarantee you would not have unknowns.

All the best,