Cannot deserialize YAML sets

[FichteFoll]

I was trying to deserialize a mapping of strings to sets and was met with the following error:

com.fasterxml.jackson.databind.exc.MismatchedInputException: Cannot deserialize value of type `java.util.HashSet<java.lang.String>` from Object value (token `JsonToken.START_OBJECT`)
 at [Source: (BufferedReader); line: 2, column: 4] (through reference chain: java.util.LinkedHashMap["A"])

YAML source:

# block-style mapping (=set)
A: !!set
  ? A:A
  ? A:B

# flow mapping (=set)
B: !!set {B:A:, B:B:}

Simplified test code:

    public Map<String, Set<String>> readConfig(Path path) throws IOException {
        var yamlMapper = new ObjectMapper(new YAMLFactory());
        try (var reader = Files.newBufferedReader(path)) {
            return yamlMapper.readValue(reader, new TypeReference<>() {});
        }
    }