stanfordnlp/CoreNLP · error · UnsupportedOperationException

Unknown key

Error message

Unknown key <key>

What it means

CoreLabel.initFromStrings throws UnsupportedOperationException when a key string cannot be mapped to a known CoreAnnotation class via AnnotationLookup.toCoreKey and the key is non-null. Only annotation names in the lookup table are accepted.

Solutions

1. Use exact registered key names (e.g. "Text", "PartOfSpeech", "Lemma", "NamedEntityTag", "Word", "Tag", "Answer").
2. Check the typo against AnnotationLookup's key table in the version you are using.
3. Pre-map custom column names to known keys before constructing the CoreLabel.
4. If the key is genuinely new, add a CoreAnnotation class registered in AnnotationLookup or set the value via label.set after construction.

Example fix

// before
new CoreLabel(new String[]{"pos"}, new String[]{"NN"}); // Unknown key pos
// after
new CoreLabel(new String[]{"PartOfSpeech"}, new String[]{"NN"});

for (String key : keys) {
  if (key != null && AnnotationLookup.toCoreKey(key) == null) {
    throw new IllegalArgumentException("Unrecognized annotation key: " + key);
  }
}

try {
  return new CoreLabel(keys, values);
} catch (UnsupportedOperationException e) {
  if (e.getMessage().startsWith("Unknown key")) {
    log.warn("Skipping unknown key; available keys are AnnotationLookup entries");
    return null; // or remap and retry
  }
  throw e;
}

When it happens

Trigger: Constructing CoreLabel from strings with a key like "pos-tag" or a custom column name that is not a registered annotation key; typos in standard keys (e.g. "PartOfSpeach"); feeding serialized label maps from a different CoreNLP version with renamed keys.

Common situations: Custom TSV/CoNLL headers not matching CoreNLP's expected key names; migrating label data between library versions where annotation names changed; hand-written key arrays using lowercase or shorthand names.

Understand the failure class

Background: Invalid enum value errors: "Unknown type", "Invalid scope", "must be one of" — when a string is not on the library's allowed list — this error's family across 23 libraries.

Related errors

• Unexpected failure to instantiate - is your key class fancy?
• Argument array lengths differ
• CORE: CoreLabel.initFromStrings: Can't handle
• CORE: CoreLabel.initFromStrings: Bad type for
• Unknown annotation key:

AI-assisted analysis of stanfordnlp/CoreNLP@1b7edd19c4 (2026-09-10). Data as JSON: /api/errors/fbb4466a327b1194. Report an issue: GitHub.

Appendix: source

Thrown at src/edu/stanford/nlp/ling/CoreLabel.java:188

  public static final Map<Class<? extends GenericAnnotation<String>>, String> genericValues = Generics.newHashMap();


  @SuppressWarnings({"unchecked", "rawtypes"})
  private void initFromStrings(String[] keys, String[] values) {
    if (keys.length != values.length) {
      throw new UnsupportedOperationException("Argument array lengths differ: " +
              Arrays.toString(keys) + " vs. " + Arrays.toString(values));
    }
    for (int i = 0; i < keys.length; i++) {
      String key = keys[i];
      String value = values[i];
      Class coreKeyClass = AnnotationLookup.toCoreKey(key);

      //now work with the key we got above
      if (coreKeyClass == null) {
        if (key != null) {
          throw new UnsupportedOperationException("Unknown key " + key);
        }
      } else {
        try {
          Class<?> valueClass = AnnotationLookup.getValueType(coreKeyClass);
          if(valueClass.equals(String.class)) {
            this.set(coreKeyClass, values[i]);
          } else if(valueClass == Integer.class) {
            this.set(coreKeyClass, Integer.parseInt(values[i]));
          } else if(valueClass == Double.class) {
            this.set(coreKeyClass, Double.parseDouble(values[i]));
          } else if(valueClass == Long.class) {
            this.set(coreKeyClass, Long.parseLong(values[i]));
          } else if (valueClass == Boolean.class) {
            this.set(coreKeyClass, Boolean.parseBoolean(values[i]));
          } else if (coreKeyClass == CoreAnnotations.CoNLLUFeats.class) {
            this.set(coreKeyClass, new CoNLLUFeatures(values[i]));
          } else {
            throw new UnsupportedOperationException("CORE: CoreLabel.initFromStrings: " +

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