stanfordnlp/CoreNLP · error · IllegalArgumentException

Expected CoreLabel's to have text

Error message

Expected CoreLabel's to have text

What it means

spanString() needs each leaf's CoreLabel.word() to hold the token text; without it the surface form cannot be reconstructed. When the first leaf is a CoreLabel but its word field is null, the library throws this IllegalArgumentException.

Solutions

1. Set the word on each leaf: ((CoreLabel) leaf.label()).setWord(tokenText) before calling spanString()
2. Run trees through LexedTokenFactory/tokenization that populates WordAnnotation, or re-tokenize with CoreNLP tokenize step
3. Use setWord/setValue consistently when constructing CoreLabels manually

Example fix

// before
CoreLabel cl = new CoreLabel(); cl.setTag("NN");
leaf.setLabel(cl); tree.spanString(); // throws
// after
cl.setWord("dog"); leaf.setLabel(cl); tree.spanString();

CoreLabel first = (CoreLabel) tree.getLeaves().get(0).label(); if (first.word() == null) throw new IllegalStateException("leaf CoreLabels missing word");

boolean leavesHaveWords(Tree t) { return t.getLeaves().stream().allMatch(l -> l.label() instanceof CoreLabel && ((CoreLabel) l.label()).word() != null); }

try { return tree.spanString(); } catch (IllegalArgumentException e) { leaves.forEach(l -> ((CoreLabel) l.label()).setWord(l.value())); return tree.spanString(); }

When it happens

Trigger: Calling spanString() on a tree whose leaves are CoreLabels that were constructed without setting the word (e.g. only tag/value set, or labels created by factories that never populate WordAnnotation).

Common situations: Programmatically built trees where CoreLabels were created empty and only value() was set; tokenization done outside CoreNLP so WordAnnotation was never applied; trees whose leaves carry only part-of-speech info.

Understand the failure class

Background: "must not be empty", "cannot be empty" — required-field validation errors across open-source libraries — this error's family across 41 libraries.

Related errors

• Expected leaves to be CoreLabels
• Expected CoreLabel's to have after() text
• Expected CoreLabels in the trees
• CORE: CoreLabel.initFromStrings: Can't handle
• CORE: CoreLabel.initFromStrings: Bad type for…

AI-assisted analysis of stanfordnlp/CoreNLP@1b7edd19c4 (2026-09-10). Data as JSON: /api/errors/6a9fa1c068c7442f. Report an issue: GitHub.

Appendix: source

Thrown at src/edu/stanford/nlp/trees/Tree.java:1021

    StringWriter sw = new StringWriter();
    pennPrint(new PrintWriter(sw));
    return sw.toString();
  }

  /**
   * Return String of leaves spanned by this tree assuming they are CoreLabel's
   * Throws an IllegalArgumentException if the leaves are not CoreLabels that contain
   * text info as in the typical use case of a Tree generated by a pipeline
   *
   * @return The text of the span of this Tree
   */
  public String spanString() {
    // check this Tree supports this method by having properly populated CoreLabel's
    List<Tree> leaves = this.getLeaves();
    if (!(leaves.get(0).label() instanceof CoreLabel)) {
      throw new IllegalArgumentException("Expected leaves to be CoreLabels");
    } else if (((CoreLabel) leaves.get(0).label()).word() == null) {
      throw new IllegalArgumentException("Expected CoreLabel's to have text");
    } else if (((CoreLabel) leaves.get(0).label()).after() == null) {
      throw new IllegalArgumentException("Expected CoreLabel's to have after() text");
    }
    List<CoreLabel> coreLabels = this.getLeaves().stream().map(l -> ((CoreLabel) l.label())).collect(Collectors.toList());
    // reconstruct original String from CoreLabel fields
    String spanString = coreLabels.subList(0, Math.max(0, coreLabels.size()-1)).stream().map(
            cl -> cl.word()+cl.after()).collect(Collectors.joining(""));
    // don't add the after of the last word
    spanString += coreLabels.get(coreLabels.size()-1).word();
    return spanString;
  }


  /**
   * Print the tree as done in Penn Treebank merged files.
   * The formatting should be exactly the same, but we don't print the
   * trailing whitespace found in Penn Treebank trees.
   * The tree is printed to {@code System.out}. The basic deviation

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