mongodb/node-mongodb-native · error · MongoInvalidArgumentError

Could not serialize ns info to BSON

Error message

Could not serialize ns info to BSON

What it means

Thrown while building a client bulk write command when BSON.serialize() fails on an nsInfo entry ({ ns: <namespace string > }). The original error is attached as `cause`. Because nsInfo only holds the namespace string, a failure here almost always means the namespace string itself is malformed or contains an unserializable (non-string) type.

Source

Thrown at src/operations/client_bulk_write/command_builder.ts:174

        }
      } else {
        // The namespace is not already in the nsInfo so we will set it in the map, and
        // construct our nsInfo and ops documents and buffers.
        namespaces.set(ns, currentNamespaceIndex);
        const nsInfo = { ns: ns };
        const operation = buildOperation(
          model,
          currentNamespaceIndex,
          this.pkFactory,
          this.options
        );
        let nsInfoBuffer;
        let operationBuffer;
        try {
          nsInfoBuffer = BSON.serialize(nsInfo);
          operationBuffer = BSON.serialize(operation);
        } catch (cause) {
          throw new MongoInvalidArgumentError(`Could not serialize ns info to BSON`, { cause });
        }

        validateBufferSize('nsInfo', nsInfoBuffer, maxBsonObjectSize);
        validateBufferSize('ops', operationBuffer, maxBsonObjectSize);

        // Check if the operation and nsInfo buffers can fit in the command. If they
        // can, then add the operation and nsInfo to their respective document
        // sequences and increment the current length as long as the ops don't exceed
        // the maxWriteBatchSize.
        if (
          commandLength + nsInfoBuffer.length + operationBuffer.length < maxMessageSizeBytes &&
          command.ops.documents.length < maxWriteBatchSize
        ) {
          // Pushing to the ops document sequence returns the total byte length of the document sequence.
          commandLength =
            MESSAGE_OVERHEAD_BYTES +
            command.nsInfo.push(nsInfo, nsInfoBuffer) +
            command.ops.push(operation, operationBuffer);

View on GitHub (pinned to 3366c21a63)

Solutions

1. Ensure every model.namespace is a non-empty 'database.collection' string.
2. Validate that namespace variables are defined and are strings before constructing models.
3. Inspect err.cause for the exact BSON encoding failure and trace which model produced it.

Example fix

// before
await client.bulkWrite([{
  namespace: undefined, // throws nsInfo serialization
  name: 'insertOne',
  document: { a: 1 }
}]);

// after
await client.bulkWrite([{
  namespace: 'mydb.mycoll',
  name: 'insertOne',
  document: { a: 1 }
}]);

function validNs(ns) {
  return typeof ns === 'string' && /^[^.]+\.[^.]+$/.test(ns);
}
models.forEach(m => { if (!validNs(m.namespace)) throw new Error('Bad namespace'); });

function isNamespaceString(v): v is string { return typeof v === 'string' && v.includes('.'); }

try { await client.bulkWrite(models); } catch (e) {
  if (e instanceof MongoInvalidArgumentError && /ns info/.test(e.message)) {
    console.error('nsInfo serialization failed:', e.cause);
  }
  throw e;
}

When it happens

Trigger: A model.namespace that is undefined, null, an object, or a string with embedded non-BSON characters; constructing models dynamically and leaving namespace unset.

Common situations: Missing namespace field in a programmatic model; building namespace from undefined variables (resulting in 'undefined'); feeding non-string values from config.

Related errors

• Could not serialize operation to BSON
• Document is larger than the maximum size ${this.s.maxBsonObj
• Document is larger than the maximum size ${this.s.maxBsonObj
• Client bulk write operation ${name} of length ${buffer.lengt
• Client bulk write update models must only contain atomic mod

AI-assisted analysis of mongodb/node-mongodb-native@3366c21a63 (2026-08-04). Data as JSON: /data/errors/37c4171571dd0e72.json. Report an issue: GitHub.