stanfordnlp/CoreNLP · error · RuntimeException

Bad line:

Error message

Bad line:

What it means

MalletReaderAndWriter reads CRF training/test data in Mallet's 'word TAB label' format. For each non-blank line it takes the last space as the word/label separator; if a line contains no space at all it cannot be split into a token and a label, so it throws this RuntimeException. It is a data-format error in the input file, not a library bug.

Solutions

  1. Fix the offending input line so it contains 'word label' separated by a space (the LAST space splits word from label).
  2. Remove or blank out empty/junk lines that contain non-whitespace characters but no space.
  3. Verify the reader matches your file's delimiter; if your file is tab-separated, pre-convert tabs to spaces or use a reader configured for your format.
  4. Wrap the read loop and log the failing line to locate it, then correct it in the source corpus.

Example fix

// before (bad input line)
Hello
// after
Hello O
Defensive patterns

Strategy: validation

Validate before calling

for (String line : lines) {
  if (line.trim().isEmpty()) continue;
  if (line.lastIndexOf(" ") < 0) {
    throw new IllegalArgumentException("Input line has no 'word label' separator: " + line);
  }
}

Type guard

boolean isMalletLine(String line) { return line != null && line.trim().length() > 0 && line.lastIndexOf(" ") >= 0; }

Try / catch

try {
  reader.apply(iter);
} catch (RuntimeException e) {
  if (e.getMessage() != null && e.getMessage().startsWith("Bad line:")) {
    log.error("Fix CRF input format: " + e.getMessage());
  } else throw e;
}

Prevention

When it happens

Trigger: Calling MalletReaderAndWriter.apply() (via a ColumnDocumentReaderAndWriter-style pipeline) with an input line that has no space character, e.g. a single-token line like 'Hello' instead of 'Hello O', or a line separated by tabs instead of spaces.

Common situations: Converting Mallet or custom corpora to Stanford NLP CRF format; lines with only a word and no gold label; trailing junk lines in a .txt training file; using TAB separators after a script changed the delimiter.

Understand the failure class

Background: "Invalid ... format", "must be in format X", "does not look like a ..." — invalid argument format errors across CLI tools and libraries — this error's family across 17 libraries.

Related errors

AI-assisted analysis of stanfordnlp/CoreNLP@1b7edd19c4 (2026-09-10). Data as JSON: /api/errors/cd32c7e1f2cbc606. Report an issue: GitHub.

Appendix: source

Thrown at src/edu/stanford/nlp/sequences/MalletReaderAndWriter.java:67

  int num = 0;
  private class MalletDocParser implements Serializable, Function<String,List<CoreLabel>> {
    private static final long serialVersionUID = -6211332661459630572L;
    @Override
    public List<CoreLabel> apply(String doc) {

      if (num % 1000 == 0) { log.info("["+num+"]"); }
      num++;
      
      List<CoreLabel> words = new ArrayList<>();
      
      String[] lines = doc.split("\n");

      for (String line : lines) {
        if (line.trim().length() < 1)
          continue;
        int idx = line.lastIndexOf(" ");
        if (idx < 0)
          throw new RuntimeException("Bad line: " + line);
        CoreLabel wi = new CoreLabel();
        wi.setWord(line.substring(0, idx));
        wi.set(CoreAnnotations.AnswerAnnotation.class, line.substring(idx + 1));
        wi.set(CoreAnnotations.GoldAnswerAnnotation.class, line.substring(idx + 1));
        words.add(wi);
      }
      return words;
    }
  }
  
  @Override
  public void printAnswers(List<CoreLabel> doc, PrintWriter out) {
    for (CoreLabel wi : doc) {
      String answer = wi.get(CoreAnnotations.AnswerAnnotation.class);
      String goldAnswer = wi.get(CoreAnnotations.GoldAnswerAnnotation.class);
      out.println(wi.word() + "\t" + goldAnswer + "\t" + answer);
    }
    out.println();

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