What causes "ancestor: height cannot be negative" in stanfordnlp/CoreNLP?

Clamp the height: if (h < 0) return null; before calling ancestor

stanfordnlp/CoreNLP · error · IllegalArgumentException

ancestor: height cannot be negative

Error message

ancestor: height cannot be negative

What it means

Argument-validation guard in Tree.ancestor(int height, Tree root): the height parameter counts how many levels up to walk (0 = this node, 1 = parent, ...), so a negative value has no meaning and is rejected immediately before any traversal begins. It indicates the caller passed height < 0 — the faulty input is the height argument itself, not tree structure.

Solutions

Clamp the height: if (h < 0) return null; before calling ancestor
Compute the level difference with depth/pathLength checks and only call ancestor when height >= 0
Fix the index arithmetic producing the negative height

Example fix

// before
Tree anc = node.ancestor(node.depth() - target.depth(), root); // can be negative
// after
int h = node.depth() - target.depth();
Tree anc = h >= 0 ? node.ancestor(h, root) : null;

Defensive patterns

Strategy: validation

Validate before calling

int h = computedHeight; if (h < 0) return null; Tree anc = node.ancestor(h, root);

Type guard

boolean validAncestorHeight(int h) { return h >= 0; }

Try / catch

try { return node.ancestor(h, root); } catch (IllegalArgumentException e) { return null; }

Prevention

Clamp or validate computed heights before calling ancestor
Remember height 0 returns the node itself
Verify depth arithmetic sign when comparing node levels

When it happens

Trigger: Calling ancestor(-1, root) or similar, usually from a loop computing relative height as `someIndex - currentIndex` where the index arithmetic can go negative.

Common situations: Off-by-one arithmetic in sentence-segmentation or clause-level extraction code; passing a difference of depths that is negative because the 'ancestor' is actually a descendant.

Related errors

AI-assisted analysis of stanfordnlp/CoreNLP@1b7edd19c4 (2026-09-10). Data as JSON: /api/errors/8c878f80c22b0ce3. Report an issue: GitHub.

Appendix: source

Thrown at src/edu/stanford/nlp/trees/Tree.java:2282

    for (Tree kid : kids) {
      size += kid.size();
    }
    return size;
  }

  /**
   * Return the ancestor tree node {@code height} nodes up from the current node.
   *
   * @param height How many nodes up to go. A parameter of 0 means return
   *               this node, 1 means to return the parent node and so on.
   * @param root The root node that this Tree is embedded under
   * @return The ancestor at height {@code height}.  It returns null
   *         if it does not exist or the tree implementation does not keep track
   *         of parents
   */
  public Tree ancestor(int height, Tree root) {
    if (height < 0) {
      throw new IllegalArgumentException("ancestor: height cannot be negative");
    }
    if (height == 0) {
      return this;
    }
    Tree par = parent(root);
    if (par == null) {
      return null;
    }
    return par.ancestor(height - 1, root);
  }


  private static class TreeIterator implements Iterator<Tree> {

    private final List<Tree> treeStack;

    protected TreeIterator(Tree t) {
      treeStack = new ArrayList<>();

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