first argument expected to be a fs.File instance, got %T ins

Error message

first argument expected to be a fs.File instance, got %T instead

What it means

Error "first argument expected to be a fs.File instance, got %T instead" thrown in grafana/k6.

Source

Thrown at internal/js/modules/k6/experimental/csv/module.go:104

	// options holds the parser's as provided by the user.
	options options

	// vu is the VU instance that owns this module instance.
	vu modules.VU
}

// parseSharedArrayNamePrefix is the prefix used for the shared array names created by the Parse function.
const parseSharedArrayNamePrefix = "csv.parse."

// Parse parses the provided CSV file, and returns a promise that resolves to a shared array
// containing the parsed data.
func (mi *ModuleInstance) Parse(file sobek.Value, options sobek.Value) (*sobek.Promise, error) {
	rt := mi.vu.Runtime()

	// 1. Make sure the Sobek object is a fs.File (Sobek operation)
	var fileObj fs.File
	if err := mi.vu.Runtime().ExportTo(file, &fileObj); err != nil {
		return nil, fmt.Errorf("first argument expected to be a fs.File instance, got %T instead", file)
	}

	parserOptions := newDefaultParserOptions()
	if options != nil {
		var err error
		parserOptions, err = newParserOptionsFrom(options.ToObject(rt))
		if err != nil {
			return nil, fmt.Errorf("failed to interpret the provided Parser options; reason: %w", err)
		}
	}

	r, err := NewReaderFrom(fileObj.ReadSeekStater, parserOptions)
	if err != nil {
		return nil, fmt.Errorf("failed to create a new parser; reason: %w", err)
	}

	underlyingSharedArrayName, err := buildSharedArrayName(fileObj, parserOptions)
	if err != nil {

View on GitHub (pinned to 93accf6570)

When it happens

Trigger: Thrown at internal/js/modules/k6/experimental/csv/module.go:104 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of grafana/k6@93accf6570 (2026-08-15). Data as JSON: /api/errors/bc539d526e1c645e. Report an issue: GitHub.