grafana/k6 · error

method to invoke cannot be empty

Error message

method to invoke cannot be empty

What it means

buildInvokeRequest validates the method string right after the connection check: an empty method name cannot identify any RPC, so it is rejected before the method-descriptor lookup. (A non-empty but unknown method gets a different error: 'method %q not found in file descriptors'.)

Solutions

1. Pass the fully-qualified method as 'package.Service/Method' (leading '/' is added automatically if missing).
2. When method names come from data, validate them first: if (!method) throw new Error('method name required').
3. If you get 'method not found in file descriptors' instead, the name is fine but the proto was not loaded - check client.load().

Example fix

// before
client.invoke('', { name: 'k6' }); // throws

// after
client.invoke('hello.HelloService/SayHello', { name: 'k6' });

function invokeGrpc(client, method, req) {
  if (typeof method !== 'string' || method.trim() === '') {
    throw new TypeError(`gRPC method must be a non-empty 'package.Service/Method' string`);
  }
  return client.invoke(method, req);
}

const isGrpcMethod = (m) => typeof m === 'string' && m.trim().length > 0;

try {
  client.invoke(method, req);
} catch (e) {
  if (/cannot be empty/.test(e.message)) {
    throw new Error(`method name was empty - check the config source: ${JSON.stringify(methodCfg)}`);
  }
  throw e;
}

When it happens

Trigger: client.invoke('', req) with a literal empty string, or a computed method name that is empty because of a typo/failed lookup (e.g. methods[i] where i is out of range, or a config field that is missing).

Common situations: Building the method name from config or a loop over an empty array; refactoring that renames a constant to undefined; passing the request payload and method in swapped order so the first argument is an object coerced oddly; strings with only whitespace still count as non-empty and fail later at the descriptor lookup.

Understand the failure class

Background: "Must be a positive integer", "Invalid value", "Unsupported": the invalid-argument-value error family, when a library rejects the value you pass — this error's family across 35 libraries.

Related errors

• no gRPC connection, you must call connect first
• a function is expected as the second argument of…
• canceled by client (k6)
• cannot be null or undefined
• empty name provided to SharedArray's constructor

AI-assisted analysis of grafana/k6@8d06114777 (2026-08-18). Data as JSON: /api/errors/031edb153fdd35f0. Report an issue: GitHub.

Appendix: source

Thrown at internal/js/modules/k6/grpc/client.go:390

}

// buildInvokeRequest creates a new InvokeRequest from the given method name, request object and parameters
func (c *Client) buildInvokeRequest(
	method string,
	req sobek.Value,
	params sobek.Value,
) (grpcext.InvokeRequest, error) {
	grpcReq := grpcext.InvokeRequest{}

	state := c.vu.State()
	if state == nil {
		return grpcReq, common.NewInitContextError("invoking RPC methods in the init context is not supported")
	}
	if c.conn == nil {
		return grpcReq, errors.New("no gRPC connection, you must call connect first")
	}
	if method == "" {
		return grpcReq, errors.New("method to invoke cannot be empty")
	}
	if method[0] != '/' {
		method = "/" + method
	}
	methodDesc := c.mds[method]
	if methodDesc == nil {
		return grpcReq, fmt.Errorf("method %q not found in file descriptors", method)
	}

	p, err := newCallParams(c.vu, params)
	if err != nil {
		return grpcReq, fmt.Errorf("invalid GRPC's client.invoke() parameters: %w", err)
	}

	// k6 GRPC Invoke's default timeout is 2 minutes
	if p.Timeout == time.Duration(0) {
		p.Timeout = 2 * time.Minute
	}

View on GitHub (pinned to 8d06114777)