grafana/k6 · error

invalid type %T, expected string or ArrayBuffer

Error message

invalid type %T, expected string or ArrayBuffer

What it means

The http.file() helper only accepts string or sobek.ArrayBuffer as the data argument (anything else — object, number, null — hits the default case). It is a strict type validation on the first argument of http.file used to build multipart upload data.

Source

Thrown at js/modules/k6/http/file.go:40

}

// File returns a FileData object.
func (mi *ModuleInstance) file(data any, args ...string) (*FileData, error) {
	// supply valid default if filename and content-type are not specified
	fname, ct := fmt.Sprintf("%d", time.Now().UnixNano()), "application/octet-stream"

	if len(args) > 0 {
		fname = args[0]

		if len(args) > 1 {
			ct = args[1]
		}
	}

	switch data.(type) {
	case string, sobek.ArrayBuffer:
	default:
		return nil, fmt.Errorf("invalid type %T, expected string or ArrayBuffer", data)
	}

	return &FileData{
		Data:        data,
		Filename:    fname,
		ContentType: ct,
	}, nil
}

View on GitHub (pinned to 01ffac6f24)

Solutions

1. Pass file contents as a string (e.g. from open(...).read()) or an ArrayBuffer
2. Example: http.file(open('./data.bin').read(), 'data.bin')

When it happens

Trigger: Thrown at js/modules/k6/http/file.go:40 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of grafana/k6@01ffac6f24 (2026-08-18). Data as JSON: /api/errors/32d401b12243859f. Report an issue: GitHub.