stanfordnlp/CoreNLP · error · IllegalArgumentException

Expected tree labels to be CoreLabel

Error message

Expected tree labels to be CoreLabel

What it means

CreateTransitionSequence.createTransitionSequenceHelper() derives the head words of a binarized tree node and its two children to emit BinaryTransitions; it requires all three labels to be CoreLabel instances. If any label is another Label implementation, it throws this IllegalArgumentException, because only CoreLabel can carry the HeadWordLabelAnnotation it reads next.

Solutions

  1. Preprocess trees with the parser's TreeBinarizer/CoreLabel conversion (as ShiftReduceParser.train does) before generating transition sequences.
  2. Ensure the tree is binarized so children()[0] and children()[1] exist.
  3. Use Trees.toTree / the same TreebankLangParserParams pipeline that produced the training data.
  4. In unit tests, build trees with CoreLabelFactory-labeled nodes.

Example fix

// before
List<Transition> trans = CreateTransitionSequence.createTransitionSequence(rawTree, false);
// after
Tree binarized = Binarizer.binarez(rawTree); // or run parser's preprocessing
List<Transition> trans = CreateTransitionSequence.createTransitionSequence(binarized, false);
Defensive patterns

Strategy: validation

Validate before calling

boolean ok = tree.label() instanceof CoreLabel
    && tree.children().length >= 2
    && tree.children()[0].label() instanceof CoreLabel
    && tree.children()[1].label() instanceof CoreLabel;
if (!ok) throw new IllegalArgumentException("Tree must be binarized with CoreLabel nodes");

Type guard

boolean isBinarizedCoreLabelTree(Tree t) {
    return t.label() instanceof CoreLabel
        && t.children().length == 2
        && t.children()[0].label() instanceof CoreLabel
        && t.children()[1].label() instanceof CoreLabel;
}

Try / catch

try {
    List<Transition> trans = CreateTransitionSequence.createTransitionSequence(tree, false);
} catch (IllegalArgumentException e) {
    if (e.getMessage().equals("Expected tree labels to be CoreLabel")) {
        // binarize + convert labels, then retry
    }
}

Prevention

When it happens

Trigger: Calling createTransitionSequence on trees not preprocessed into CoreLabel-backed binarized form — e.g. raw trees from a different parser or trees whose children array has fewer than 2 non-null children (children()[1] null).

Common situations: Training the shift-reduce parser with a custom treebank loaded without the parser's tree normalization, or passing non-binarized/unheaded trees directly to the transition-sequence generator.

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

AI-assisted analysis of stanfordnlp/CoreNLP@1b7edd19c4 (2026-09-10). Data as JSON: /api/errors/d7ec7ca8ea1a2dec. Report an issue: GitHub.

Appendix: source

Thrown at src/edu/stanford/nlp/parser/shiftreduce/CreateTransitionSequence.java:75

        createTransitionSequenceHelper(transitions, tree, compoundUnary, rootOnlyStates);
        transitions.add(new CompoundUnaryTransition(labels, isRoot));
      } else {
        createTransitionSequenceHelper(transitions, tree.children()[0], compoundUnary, rootOnlyStates);
        transitions.add(new UnaryTransition(tree.label().value(), isRoot));
      }
    } else if (tree.children().length == 2) {
      createTransitionSequenceHelper(transitions, tree.children()[0], compoundUnary, rootOnlyStates);
      createTransitionSequenceHelper(transitions, tree.children()[1], compoundUnary, rootOnlyStates);

      // This is the tricky part... need to decide if the binary
      // transition is a left or right transition.  This is done by
      // looking at the existing heads of this node and its two
      // children.  The expectation is that the tree already has heads
      // assigned; otherwise, exception is thrown
      if (!(tree.label() instanceof CoreLabel) || 
          !(tree.children()[0].label() instanceof CoreLabel) ||
          !(tree.children()[1].label() instanceof CoreLabel)) {
        throw new IllegalArgumentException("Expected tree labels to be CoreLabel");
      }
      CoreLabel label = (CoreLabel) tree.label();
      CoreLabel leftLabel = (CoreLabel) tree.children()[0].label();
      CoreLabel rightLabel = (CoreLabel) tree.children()[1].label();
      CoreLabel head = label.get(TreeCoreAnnotations.HeadWordLabelAnnotation.class);
      CoreLabel leftHead = leftLabel.get(TreeCoreAnnotations.HeadWordLabelAnnotation.class);
      CoreLabel rightHead = rightLabel.get(TreeCoreAnnotations.HeadWordLabelAnnotation.class);
      if (head == null || leftHead == null || rightHead == null) {
        throw new IllegalArgumentException("Expected tree labels to have their heads assigned.  Failed at: " + tree);
      }
      boolean isRoot = rootOnlyStates.contains(tree.label().value());
      if (head == leftHead) {
        transitions.add(new BinaryTransition(tree.label().value(), BinaryTransition.Side.LEFT, isRoot));
      } else if (head == rightHead) {
        transitions.add(new BinaryTransition(tree.label().value(), BinaryTransition.Side.RIGHT, isRoot));
      } else {
        throw new IllegalArgumentException("Heads were incorrectly assigned: tree's head is not matched to either the right or left head");
      }

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