Russian language in CouchDB

Question

I try to run query in CouchDB 2.3.1 with russian language data using regex and get nothing:

{
   "selector": {
      "class_name": "cat.patients",
      "name": {
         "$regex": "(?i)ИВАНОВА"
      }
   }
}

But this query works:

{
   "selector": {
      "class_name": "cat.patients",
      "name": {
         "$regex": "Иванова"
      }
   }
}

What is the problem here?