What causes "Unknown language " in stanfordnlp/CoreNLP?

Set the `language` property to a known value, e.g. -language english (or the appropriate UD language name)

stanfordnlp/CoreNLP · error · IllegalArgumentException

Unknown language <props.containsKey("language")>

Error message

Unknown language <props.containsKey("language")>

What it means

DependencyParser's Config.setProperties() resolves the `language` property via getLanguage(). If no language property is given (or an unrecognized name is given) the resolved `language` ends up null, and this IllegalArgumentException is thrown. It indicates the parser could not determine which language configuration (treebank language pack, head rules, etc.) to use.

Solutions

Set the `language` property to a known value, e.g. -language english (or the appropriate UD language name)
Check accepted values in Config.getLanguage() / the Language enum and fix spelling/case
If constructing Config programmatically, pass props.put("language", "english") before creating the parser

Example fix

// before
Properties props = new Properties();
DependencyParser parser = DependencyParser.train(props); // language missing
// after
Properties props = new Properties();
props.setProperty("language", "english");
DependencyParser parser = DependencyParser.train(props);

Defensive patterns

Strategy: validation

Validate before calling

Properties props = new Properties();
if (props.getProperty("language") == null) props.setProperty("language", "english");

Try / catch

try {
  DependencyParser parser = DependencyParser.train(props);
} catch (IllegalArgumentException e) {
  props.setProperty("language", "english");
  DependencyParser parser = DependencyParser.train(props);
}

Prevention

Always set `language` explicitly in parser properties
Verify language names against Config.getLanguage()'s supported set
Note the exception prints containsKey(true/false), not the bad value - log your props yourself

When it happens

Trigger: Instantiating DependencyParser/Config with a Properties object whose `language` key is missing or whose value getLanguage() doesn't recognize (note the message prints props.containsKey("language"), so it confusingly shows true/false).

Common situations: Typos in -language value (e.g. 'english' vs 'englishsd'); building a parser programmatically without setting language; copying config from another parser (e.g. stanford-parser) that used different language keys.

Understand the failure class

Background: "Invalid value" and "allowed values are" config errors: what your library rejected and how to fix it — this error's family across 41 libraries.

Related errors

AI-assisted analysis of stanfordnlp/CoreNLP@1b7edd19c4 (2026-09-10). Data as JSON: /api/errors/af709e25439af978. Report an issue: GitHub.

Appendix: source

Thrown at src/edu/stanford/nlp/parser/nndep/Config.java:260

    saveIntermediate = PropertiesUtils.getBool(props, "saveIntermediate", saveIntermediate);
    unlabeled = PropertiesUtils.getBool(props, "unlabeled", unlabeled);
    cPOS = PropertiesUtils.getBool(props, "cPOS", cPOS);
    noPunc = PropertiesUtils.getBool(props, "noPunc", noPunc);
    doWordEmbeddingGradUpdate = PropertiesUtils.getBool(props, "doWordEmbeddingGradUpdate", doWordEmbeddingGradUpdate);

    // Runtime parsing options
    sentenceDelimiter = PropertiesUtils.getString(props, "sentenceDelimiter", sentenceDelimiter);
    tagger = PropertiesUtils.getString(props, "tagger.model", tagger);

    String escaperClass = props.getProperty("escaper");
    escaper = escaperClass != null ? ReflectionLoading.loadByReflection(escaperClass) : null;

    // Language options
    language = props.containsKey("language")
               ? getLanguage(props.getProperty("language"))
               : language;
    if (language == null) {
      throw new IllegalArgumentException("Unknown language " + props.containsKey("language"));
    }
    if (language.params == null) {
      throw new IllegalArgumentException("Language " + language + " does not support parsing!");
    }
    tlp = language.params.treebankLanguagePack();
    preTokenized = PropertiesUtils.getBool(props, "tokenized", preTokenized);

    // if a tlp was specified go with that
    String tlpCanonicalName = props.getProperty("tlp");
    if (tlpCanonicalName != null) {
      try {
        tlp = ReflectionLoading.loadByReflection(tlpCanonicalName);
        System.err.println("Loaded TreebankLanguagePack: "+tlpCanonicalName);
      } catch (Exception e) {
        System.err.println("Error: Failed to load TreebankLanguagePack: "+tlpCanonicalName);
      }
    }
  }

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