stanfordnlp/CoreNLP · error · java.lang.IllegalArgumentException

Unknown format

Error message

Unknown format 

What it means

CoreLabel.toString(Format format) supports only a fixed set of Format enum values (e.g. VALUE, WORD, TAG, MAP, VALUE_TAG, etc.). If a caller passes a null or unhandled format value, the switch's default branch throws IllegalArgumentException("Unknown format " + format). This is a defensive guard against invalid format requests rather than a data problem.

Solutions

1. Check the format string/value passed; use only constants defined in CoreLabel.OutputFormat (or AbstractMapLabel.Format) that this version supports.
2. Null-check or default the format: Format f = fmt == null ? Format.VALUE : fmt.
3. Call the no-arg toString() instead of toString(format) when you just want the value.
4. Upgrade Stanford CoreNLP if the desired format constant exists only in newer releases.

Example fix

// before
String s = label.toString(Format.valueOf(config.get("fmt"))); // IAE if unknown/null
// after
String s = label.toString("VALUE".equals(config.get("fmt")) ? Format.VALUE : Format.VALUE);

// Allowed formats for CoreLabel.toString(Format)
Set<CoreLabel.OutputFormat> allowed = EnumSet.allOf(CoreLabel.OutputFormat.class);
if (format == null || !allowed.contains(format)) {
  format = CoreLabel.OutputFormat.VALUE; // safe default
}
String s = label.toString(format);

boolean isKnownFormat(CoreLabel.OutputFormat f) {
  return f != null && EnumSet.allOf(CoreLabel.OutputFormat.class).contains(f);
}

try {
  return label.toString(format);
} catch (IllegalArgumentException e) {
  log.warn("Unknown label format " + format + ", using VALUE");
  return label.toString(CoreLabel.OutputFormat.VALUE);
}

When it happens

Trigger: Calling label.toString(Format) with a Format value not covered by the switch — most commonly Format.valueOf on a typo'd name, a null Format after parsing a format string, or a Format constant from a different/newer CoreLabel.Format enum than this StanNLP version supports.

Common situations: Configuring an output format via a user-supplied string and passing it unvalidated to toString(Format); version skew where an older Stanford CoreNLP build lacks a newer Format enum constant that configuration references.

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

• Unknown clique: " + clique
• Sentence.toSentence: lengths differ
• Invalid arguments to
• CoreMap must have either a Calendar or DocDate annotation
• Unsupported subScoreType

AI-assisted analysis of stanfordnlp/CoreNLP@1b7edd19c4 (2026-09-10). Data as JSON: /api/errors/f2741e1e192c426e. Report an issue: GitHub.

Appendix: source

Thrown at src/edu/stanford/nlp/ling/CoreLabel.java:856

    case LEMMA_INDEX:
      buf.append(lemma());
      Integer index = this.get(CoreAnnotations.IndexAnnotation.class);
      if (index != null) {
        buf.append('-').append((index).intValue());
      }
      Integer emptyIndex = this.get(CoreAnnotations.EmptyIndexAnnotation.class);
      if (emptyIndex != null && emptyIndex != 0) {
        buf.append('.').append((emptyIndex).intValue());
      }
      break;
    case ALL:{
      for (Class en: this.keySet()) {
        buf.append(';').append(en).append(':').append(this.get(en));
      }
      break;
    }
    default:
      throw new IllegalArgumentException("Unknown format " + format);
    }
    return buf.toString();
  }

  private static final Comparator<Class<?>> asClassComparator =
          Comparator.comparing(Class::getName);

}

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