stanfordnlp/CoreNLP · error · IllegalArgumentException

Cannot put a child trie with no keys

Error message

Cannot put a child trie with no keys

What it means

TrieMap.putChildTrie throws IllegalArgumentException('Cannot put a child trie with no keys') when the computed parent trie is null — i.e., the key path used to locate the parent was empty, so there is nowhere to attach the child. The trie requires a non-empty key sequence to insert.

Solutions

1. Skip the insert when the key is empty, or treat empty key as the root value separately.
2. Sanitize input data so rules/phrases never produce zero-length keys.
3. If an empty key is meaningful, store the value in the root TrieMap's value field instead.

Example fix

// before
trie.put(tokens, value); // tokens may be empty
// after
if (tokens == null || tokens.isEmpty()) {
    return; // or handle root-level value
}
trie.put(tokens, value);

if (key == null || key.isEmpty()) {
    throw new IllegalArgumentException("cannot insert empty key into TrieMap");
}

static <K> boolean hasKey(Iterable<K> key) {
    return key != null && key.iterator().hasNext();
}

try {
    trie.put(key, value);
} catch (IllegalArgumentException e) {
    if (e.getMessage().contains("no keys")) {
        log.warn("skipped empty-key insert");
    } else throw e;
}

When it happens

Trigger: Calling putChildTrie (directly or via put) with an empty key Iterable, so the loop never descends and parentTrie remains null.

Common situations: Inserting empty token lists into a token trie (e.g., empty phrases/rules read from a config); upstream code that filters out tokens but still attempts the insert.

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

• Value cannot be null
• Invalid nextBranchIndex=
• Invalid captureGroupId=
• Invalid minMatch=
• Unsupported subScoreType

AI-assisted analysis of stanfordnlp/CoreNLP@1b7edd19c4 (2026-09-10). Data as JSON: /api/errors/a1b4c2b2ed187f8d. Report an issue: GitHub.

Appendix: source

Thrown at src/edu/stanford/nlp/ling/tokensregex/matcher/TrieMap.java:78

    TrieMap<K, V> curTrie = this;
    Iterator<K> keyIter = key.iterator();
    // go through each element
    while (keyIter.hasNext()) {
      K element = keyIter.next();
      boolean isLast = !keyIter.hasNext();
      if (curTrie.children == null) {
        curTrie.children = new ConcurrentHashMap<>();//Generics.newConcurrentHashMap();
      }
      parentTrie = curTrie;
      curTrie = curTrie.children.get(element);
      if (isLast) {
        parentTrie.children.put(element, child);
      } else if(curTrie == null){
        parentTrie.children.put(element, curTrie = new TrieMap<>());
      }
    }
    if (parentTrie == null) {
      throw new IllegalArgumentException("Cannot put a child trie with no keys");
    }
    return curTrie;
  }

  public Map<K, TrieMap<K, V>> getChildren() {
    return children;
  }

  public V getValue() {
    return value;
  }

  public boolean isLeaf() {
    return value != null;
  }


  public String toFormattedString() {

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