mongodb/node-mongodb-native · error · MongoParseError

Cannot have undefined values in key value pairs

Error message

Cannot have undefined values in key value pairs

What it means

Thrown by entriesFromString() when parsing a comma-separated key:value option string and a segment lacks the value after the colon. Used for options parsed as maps such as readPreferenceTags and authMechanismProperties. Splitting 'key:value' on the first ':' yields an undefined value when the segment is 'key' with no colon or ends with a colon. It is a MongoParseError.

Source

Thrown at src/connection_string.ts:212

}

function getUIntFromOptions(name: string, value: unknown): number {
  const parsedValue = getIntFromOptions(name, value);
  if (parsedValue < 0) {
    throw new MongoParseError(`${name} can only be a positive int value, got: ${value}`);
  }
  return parsedValue;
}

function* entriesFromString(value: string): Generator<[string, string]> {
  if (value === '') {
    return;
  }
  const keyValuePairs = value.split(',');
  for (const keyValue of keyValuePairs) {
    const [key, value] = keyValue.split(/:(.*)/);
    if (value == null) {
      throw new MongoParseError('Cannot have undefined values in key value pairs');
    }

    yield [key, value];
  }
}

class CaseInsensitiveMap<Value = any> extends Map<string, Value> {
  constructor(entries: Array<[string, any]> = []) {
    super(entries.map(([k, v]) => [k.toLowerCase(), v]));
  }
  override has(k: string) {
    return super.has(k.toLowerCase());
  }
  override get(k: string) {
    return super.get(k.toLowerCase());
  }
  override set(k: string, v: any) {
    return super.set(k.toLowerCase(), v);

View on GitHub (pinned to 3366c21a63)

Solutions

1. Ensure every tag/mproperty segment is 'key:value' with both parts non-empty.
2. Filter empty segments before joining: tags.filter(t => t).join(',').
3. Validate the built string with a regex like /^[^,]+:[^,]+(,[^,]+:[^,]+)*$/ before passing.
4. Avoid trailing/leading commas and trailing colons in templated values.

Example fix

// before
const tags = [`region:${r}`, zone ? `zone:${zone}` : ''].filter(Boolean).join(',');
// trailing segment 'zone:' if zone present without value
// after
const tags = [{ region: r }, zone ? { zone } : null]
  .filter(Boolean)
  .map(Object.entries)
  .map(([k, v]) => `${k}:${v}`)
  .join(',');
new MongoClient(uri, { readPreferenceTags: tags ? [tags] : undefined });

function buildTagString(entries: Record<string, string>[]): string {
  const valid = entries
    .flatMap(Object.entries)
    .filter(([k, v]) => k && v)
    .map(([k, v]) => `${k}:${v}`);
  if (!valid.length) return '';
  if (!/^[^,]+:[^,]+(,[^,]+:[^,]+)*$/.test(valid.join(','))) {
    throw new Error('Malformed key:value tag string');
  }
  return valid.join(',');
}

const isKeyValueSegments = (s: string): boolean =>
  s.split(',').every(seg => /^[^:]+:[^:]+$/.test(seg));

try {
  await client.connect();
} catch (e) {
  if (e instanceof MongoParseError && /undefined values in key value pairs/.test(e.message)) {
    throw new Error('A readPreferenceTags/authMechanismProperties segment is missing a value');
  }
  throw e;
}

When it happens

Trigger: URI like ?readPreferenceTags=region:us-east,zone or ?authMechanismProperties=AWS_SESSION_TOKEN: (trailing colon); a tag without a value: ?readPreferenceTags=,; malformed comma-separated entries with stray commas.

Common situations: Building readPreferenceTags dynamically and emitting a trailing comma; templating authMechanismProperties with an unset env var producing 'KEY:'; copy-paste from docs that showed a placeholder; concatenating tags with a leading/trailing comma.

Related errors

• ${name} must be either "true" or "false"
• Expected ${name} to be stringified int value, got: ${value}
• ${name} can only be a positive int value, got: ${value}
• URI option "${key}" cannot appear more than once in the conn
• URI option "${key}" cannot be specified with no value

AI-assisted analysis of mongodb/node-mongodb-native@3366c21a63 (2026-08-04). Data as JSON: /data/errors/656ee627c2ff98e6.json. Report an issue: GitHub.