"500 Internal Server Error" in /indigo/descriptors

**Steps to Reproduce**
Make request to `/indigo/descriptors` with the parameters `"acidPkaValue"` and/or `"basicPkaValue"`
```
{
"data": {
"type": "descriptor",
"attributes": {
"compound": {
"structure": "CCC",
"format": "auto"
},
"descriptors": [
"acidPkaValue",
"basicPkaValue"
]
}
}
}
```

**Expected behavior**
Correct processing of the request and return of the error description

**Actual behavior**
`Error: 500 Internal Server Error`

**Indigo/Bingo version**
1.6.1.0-g5dbb0c0f4-x86_64-linux-gnu-10.2.1

**Additional context**
The error can be reproduced even in /docs