n8n-io/n8n · error · NodeOperationError

Error: No JSON string provided.

Error message

Error: No JSON string provided.

What it means

NodeOperationError thrown by `getFilterValue` when the option `name` is truthy but its value is not a string. The handler only attempts `JSON.parse` on strings, so any other type (number, object, array) — which the UI should not produce but expressions can — lands here.

Source

Thrown at packages/@n8n/nodes-langchain/nodes/vector_store/VectorStoreMongoDBAtlas/VectorStoreMongoDBAtlas.node.ts:256

export function getFilterValue<T>(
	name: string,
	context: IExecuteFunctions | ISupplyDataFunctions,
	itemIndex: number,
): T | undefined {
	const options: IDataObject = context.getNodeParameter('options', itemIndex, {});

	if (options[name]) {
		if (typeof options[name] === 'string') {
			try {
				return JSON.parse(options[name]);
			} catch (error) {
				throw new NodeOperationError(context.getNode(), `Error: ${error.message}`, {
					itemIndex,
					description: `Could not parse JSON for ${name}`,
				});
			}
		}
		throw new NodeOperationError(context.getNode(), 'Error: No JSON string provided.', {
			itemIndex,
			description: `Could not parse JSON for ${name}`,
		});
	}

	return undefined;
}

class ExtendedMongoDBAtlasVectorSearch extends MongoDBAtlasVectorSearch {
	mongoClient: MongoClient;
	preFilter: IDataObject;
	postFilterPipeline?: IDataObject[];

	constructor(
		embeddings: EmbeddingsInterface,
		options: MongoDBAtlasVectorSearchLibArgs,
		mongoClient: MongoClient,
		preFilter: IDataObject,

View on GitHub (pinned to 5ac6606e81)

Solutions

1. If the value is already an object, pass it through `JSON.stringify` first or restructure to bypass the option field.
2. Set the option field to a JSON string literal (e.g. `{ "category": "books" }`).
3. Use an expression that returns a string: `={{ JSON.stringify($json.filter) }}`.

Example fix

// before
if (typeof options[name] === 'string') {
  // ... JSON.parse ...
}
throw new NodeOperationError(context.getNode(), 'Error: No JSON string provided.', {
  itemIndex,
  description: `Could not parse JSON for ${name}`,
});

// after: accept already-parsed objects instead of rejecting them
if (typeof options[name] === 'string') {
  try { return JSON.parse(options[name]); } catch (error) { /* ... */ }
}
if (options[name] && typeof options[name] === 'object') {
  return options[name];
}
throw new NodeOperationError(context.getNode(), `Expected a JSON string for ${name}`, { itemIndex });

// Accept either a JSON string or an already-parsed object.
function resolveFilterValue<T>(value: unknown, name: string, itemIndex: number): T | undefined {
  if (!value) return undefined;
  if (typeof value === 'string') {
    try { return JSON.parse(value) as T; }
    catch (error) { throw new NodeOperationError(/* ... */); }
  }
  if (typeof value === 'object') return value as T;
  throw new NodeOperationError(/* 'Expected JSON string or object' */);
}

function isJsonable(value: unknown): value is string | object {
  return typeof value === 'string' || (typeof value === 'object' && value !== null);
}

// No external try/catch needed; this is a synchronous shape check.
if (options[name] && typeof options[name] !== 'string' && typeof options[name] === 'object') {
  return options[name];
}
if (typeof options[name] === 'string') { /* JSON.parse path */ }
throw new NodeOperationError(context.getNode(), `Expected a JSON string for ${name}`, { itemIndex });

When it happens

Trigger: An expression bound to the pre/post filter option returns an object directly (e.g. `={{ $json.filter }}` where `filter` is already an object) instead of a JSON string; the option value was injected programmatically as a non-string.

Common situations: User passes an already-parsed object via expression and skips the JSON-string step the field expects; programmatic workflow injection setting the option to an object; typeVersion drift leaving the option as a non-string.

Related errors

• Parameter ${key} must be a string
• Collection must be a string
• Error: ${error.message}
• Index ${mongoVectorIndexName} not found
• Error connecting to ChromaDB: ${message}

AI-assisted analysis of n8n-io/n8n@5ac6606e81 (2026-08-12). Data as JSON: /api/errors/d31c6872dbb9c4e7. Report an issue: GitHub.