the 'fromLine' option must be less than the 'toLine' option;

Error message

the 'fromLine' option must be less than the 'toLine' option; got 'fromLine': %d, 'toLine': %d

What it means

The experimental CSV reader requires a non-empty, ordered read window: when both fromLine and toLine are set, fromLine must be strictly less than toLine (internal/js/modules/k6/experimental/csv/reader.go:156). Equality is rejected because fromLine is where reading starts (0-based) and toLine is the inclusive stop, so fromLine == toLine selects zero lines. Note: through the public JS APIs (new csv.Parser / csv.parse) an earlier check in newParserOptionsFrom (module.go:362) usually fires first with the message 'fromLine must be less than or equal to toLine'; the reader-level message is the underlying guard.

Source

Thrown at internal/js/modules/k6/experimental/csv/reader.go:157

	if asObjectsEnabled && skipFirstLineSet {
		return fmt.Errorf("the 'header' option cannot be enabled when 'skipFirstLine' is true")
	}

	if asObjectsEnabled && fromLineSet && options.FromLine.Int64 > 0 {
		return fmt.Errorf("the 'header' option cannot be enabled when 'fromLine' is set to a value greater than 0")
	}

	if fromLineSet && options.FromLine.Int64 < 0 {
		return fmt.Errorf("the 'fromLine' option must be greater than or equal to 0; got %d", options.FromLine.Int64)
	}

	if toLineSet && options.ToLine.Int64 < 0 {
		return fmt.Errorf("the 'toLine' option must be greater than or equal to 0; got %d", options.ToLine.Int64)
	}

	if fromLineSet && toLineSet && options.FromLine.Int64 >= options.ToLine.Int64 {
		return fmt.Errorf(
			"the 'fromLine' option must be less than the 'toLine' option; got 'fromLine': %d, 'toLine': %d",
			options.FromLine.Int64, options.ToLine.Int64,
		)
	}

	return nil
}

View on GitHub (pinned to 93accf6570)

Solutions

Make fromLine strictly smaller than toLine - to read only line 5 use { fromLine: 5, toLine: 6 }
Convert 1-based inclusive ranges as fromLine = first - 1, toLine = last
Validate the window in your config layer before constructing the parser

Example fix

// before
const parser = new csv.Parser(file, { fromLine: 5, toLine: 5 }); // error: empty window

// after
const parser = new csv.Parser(file, { fromLine: 5, toLine: 6 }); // reads exactly line 5

Defensive patterns

Strategy: validation

Validate before calling

function assertWindow(o) {
  if (o.fromLine !== undefined && o.toLine !== undefined && o.fromLine >= o.toLine) {
    throw new Error(`fromLine (${o.fromLine}) must be strictly less than toLine (${o.toLine})`);
  }
  return o;
}
const parser = new csv.Parser(file, assertWindow({ fromLine: f, toLine: t }));

Try / catch

try { new csv.Parser(file, opts); } catch (e) { if (/fromLine.*less than.*toLine|fromLine must be less than/.test(e.message)) { /* recompute bounds, retry */ } throw e; }

Prevention

Remember toLine is inclusive: a one-line window is {fromLine: n, toLine: n+1}
Compute both bounds from one source of truth and validate ordering before use

When it happens

Trigger: `{ fromLine: 5, toLine: 5 }` or `{ fromLine: 10, toLine: 2 }` passed to new csv.Parser() or csv.parse().

Common situations: Inclusive/exclusive confusion - treating toLine as exclusive and setting it equal to fromLine to read 'exactly one line'; bounds both computed from user input where start can overtake end.

Related errors

AI-assisted analysis of grafana/k6@93accf6570 (2026-08-15). Data as JSON: /api/errors/c9ef186c30e327cc. Report an issue: GitHub.