Pre-trained sentiment_twitter always returns negaive for Ru

Sorry, a total newbie here.
I just tested the ready-made docker image and tried to use sentiment_twitter model. It works just fine with English, but for any Russian input it always returns ‘negative’ whatever is written there.
What am I doing wrong?

An example of request (that returns negative)

curl -X 'POST' \
  'http://46.101.122.127:8000/model' \
  -H 'accept: application/json' \
  -H 'Content-Type: application/json' \
  -d '{
  "x": [
    "Привет!"
  ]
}'