I can't retrieve custom attribute values ​​from API

I’m trying to query my custom attribute values ​​related to Epics and Issues through APIs 2.15-GET and 2.31-GET (from the reference Taiga REST API). I can successfully get the list of custom attributes for Epics and Issues, but when requesting the values ​​for the respective Id’s, I always get the following message:

{“_error_message”: “No EpicCustomAttributesValues ​​matches the given query.”}

As an example, for the request made at

https://api.taiga.io/api/v1/issue-custom-attributes?project=2222

the result is

[{“id”: 9999, “name”: “ATTR_ITEM”, “description”: …

and, for the request made at

https://api.taiga.io/api/v1/issues/custom-attributes-values/9999’,

the above error message is displayed. Am I doing something wrong?