stanfordnlp/CoreNLP · error · IllegalArgumentException

CoreMap is actually a CoreLabel

Error message

CoreMap is actually a CoreLabel

What it means

toProtoBuilder expects a sentence-level CoreMap. Tokens (CoreLabel) are serialized separately by a dedicated method, so passing a CoreLabel here is an API misuse and throws IllegalArgumentException immediately as an error check.

Solutions

1. Use serializer.toProto(coreLabel, keysToSkip) for tokens instead of toProtoBuilder
2. Ensure the object passed to toProtoBuilder is a sentence/document-level CoreMap (Annotation), not a CoreLabel
3. Call the public toProto overloads rather than the protected builder methods

Example fix

// before
builder = serializer.toProtoBuilder(token, keys); // token is a CoreLabel -> throws
// after
CoreNLPProtos.Token tb = serializer.toProto(token, Collections.emptySet());

if (obj instanceof CoreLabel) { serializer.toProto((CoreLabel) obj, Collections.emptySet()); } else { serializer.toProto((CoreMap) obj); }

boolean isToken(Object o) { return o instanceof CoreLabel; }

try { serializer.toProtoBuilder(sentence, keys); } catch (IllegalArgumentException e) { if (e.getMessage().equals("CoreMap is actually a CoreLabel")) { serializer.toProto((CoreLabel) sentence, Collections.emptySet()); } else throw e; }

When it happens

Trigger: Calling ProtobufAnnotationSerializer.toProtoBuilder(sentence, keysToSerialize) with a CoreLabel instance — e.g. mixing up token and sentence objects, or iterating TokensAnnotation and passing each CoreLabel to the sentence builder.

Common situations: Custom serialization code that confuses Annotation (document/sentence) with CoreLabel (token); refactors changing variable types; calling protected builder methods directly instead of the typed toProto overloads.

Understand the failure class

Background: Type mismatch errors: IllegalArgumentException, TypeError and type guards across 150 open-source libraries — this error's family across 150 libraries.

Related errors

• RuntimeIOException wrapping IOException
• Failed to save classifier
• ERROR: Invalid dependency node line
• ERROR: Invalid format for dependency graph
• ERROR: Incorrect format for the serialized coref graph

AI-assisted analysis of stanfordnlp/CoreNLP@1b7edd19c4 (2026-09-10). Data as JSON: /api/errors/60e00a9346d2b648. Report an issue: GitHub.

Appendix: source

Thrown at src/edu/stanford/nlp/pipeline/ProtobufAnnotationSerializer.java:500

    }
    return builder.build();
  }

  /**
   * <p>
   *   The method to extend by subclasses of the Protobuf Annotator if custom additions are added to Tokens.
   *   In contrast to {@link ProtobufAnnotationSerializer#toProto(edu.stanford.nlp.ling.CoreLabel)}, this function
   *   returns a builder that can be extended.
   * </p>
   *
   * @param sentence The sentence to save to a protocol buffer
   * @param keysToSerialize A set tracking which keys have been saved. It's important to remove any keys added to the proto
   *                        from this set, as the code tracks annotations to ensure lossless serialization.
   */
  @SuppressWarnings("deprecation")
  protected CoreNLPProtos.Sentence.Builder toProtoBuilder(CoreMap sentence, Set<Class<?>> keysToSerialize) {
    // Error checks
    if (sentence instanceof CoreLabel) { throw new IllegalArgumentException("CoreMap is actually a CoreLabel"); }
    CoreNLPProtos.Sentence.Builder builder = CoreNLPProtos.Sentence.newBuilder();
    // Remove items serialized elsewhere from the required list
    keysToSerialize.remove(TextAnnotation.class);
    keysToSerialize.remove(NumerizedTokensAnnotation.class);
    // Required fields
    builder.setTokenOffsetBegin(getAndRegister(sentence, keysToSerialize, TokenBeginAnnotation.class));
    builder.setTokenOffsetEnd(getAndRegister(sentence, keysToSerialize, TokenEndAnnotation.class));
    // Get key set of CoreMap
    Set<Class<?>> keySet;
    if (sentence instanceof ArrayCoreMap) {
       keySet = ((ArrayCoreMap) sentence).keySetNotNull();
    } else {
      keySet = new IdentityHashSet<>(sentence.keySet());
    }
    // Tokens
    if (sentence.containsKey(TokensAnnotation.class)) {
      int tokenIndex = 0;
      for (CoreLabel tok : sentence.get(TokensAnnotation.class)) {

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