Re: property indexing and nested props?

I get the same  error "invalid value object: Attempt to add illegal key to
value object: ex:pfx" with this context:

  {
  "@context": {
    "@vocab": "http://example.org/",
    "ex": "http://example.org/",
    "prefixes" : {
      "@container": "@index",
      "@index": "ex:pfx"
    },
    "ex:pfx": {
      "@container": "@index",
      "@index": "ex:iri"
    }
  }
}