stanfordnlp/CoreNLP · error · IllegalArgumentException

Only works on CoreLabel

Error message

Only works on CoreLabel

What it means

Trees.setSentIndex only operates on trees whose leaves are CoreLabel instances, because the sentence index annotation lives on CoreLabel. If any leaf implements Label but not CoreLabel, it throws IllegalArgumentException immediately.

Solutions

1. Build or re-read the tree with a CoreLabel-producing factory such as LabeledScoredTreeFactory(CoreLabel.factory())
2. Convert existing leaves to CoreLabel by rebuilding the tree with TreeCoreAnnotations or a transform that replaces labels
3. Set sentIndex directly by iterating leaves and casting only after instanceof CoreLabel check

Example fix

// before
Tree t = Trees.readTree(ptbStr); // leaves may be StringLabel
Trees.setSentIndex(t, 5); // throws
// after
Tree t = Trees.readTree(ptbStr, new LabeledScoredTreeFactory(CoreLabel.factory()));
Trees.setSentIndex(t, 5);

for (Label leaf : tree.yield()) {
  if (!(leaf instanceof CoreLabel)) {
    throw new IllegalArgumentException("leaf " + leaf + " is not a CoreLabel");
  }
}

boolean hasCoreLeaves(Tree t) {
  return t.yield().stream().allMatch(l -> l instanceof CoreLabel);
}

try {
  Trees.setSentIndex(tree, sentIndex);
} catch (IllegalArgumentException e) {
  tree = rebuildWithCoreLabels(tree);
  Trees.setSentIndex(tree, sentIndex);
}

When it happens

Trigger: Calling Trees.setSentIndex(tree, i) on a tree built with a factory that produces non-CoreLabel leaves, e.g. LabeledScoredTreeFactory with StringLabel or Word leaves.

Common situations: Mixing tree factories: reading a tree with default (StringLabel-based) factories then attempting CoreNLP annotations; converting between treebanks that use different Label types.

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

• Cannot set from string
• CoreLabels required!
• Label argument lacks IndexAnnotation.
• No label for '" + text + "'
• Only operates on CoreLabels

AI-assisted analysis of stanfordnlp/CoreNLP@1b7edd19c4 (2026-09-10). Data as JSON: /api/errors/4ff3ab4fd2912804. Report an issue: GitHub.

Appendix: source

Thrown at src/edu/stanford/nlp/trees/Trees.java:897

      cl.setValue(l.value());
      tree.setLabel(cl);
    }

    for (Tree kid : tree.children()) {
      convertToCoreLabels(kid);
    }
  }


  /**
   * Set the sentence index of all the leaves in the tree
   * (only works on CoreLabel)
   */
  public static void setSentIndex(Tree tree, int sentIndex) {
    List<Label> leaves = tree.yield();
    for (Label leaf : leaves) {
      if (!(leaf instanceof CoreLabel)) {
        throw new IllegalArgumentException("Only works on CoreLabel");
      }
      ((CoreLabel) leaf).setSentIndex(sentIndex);
    }
  }
}

View on GitHub (pinned to 1b7edd19c4)