grafana/k6 · error

converting argument

Error message

converting argument '%v': %w

What it means

Wrap of a json.Marshal failure in the CDP argument converter: convertArgument serializes a Go-side argument (typically a float) to JSON for a Runtime.callFunctionOn invocation. Special values (Infinity, NaN) are handled as UnserializableValue beforehand, so reaching this wrap means the remaining value could not be marshaled — e.g., a float that overflows JSON representation, or an internal type json.Marshal cannot encode. The '%v' shows the offending argument.

Solutions

  1. Pass finite numeric values to evaluate/handle methods
  2. Check for computed values that became Inf or NaN through division by zero and handle them explicitly
  3. Convert exotic types to plain strings or numbers before passing them to browser evaluate calls
Defensive patterns

Strategy: try-catch

When it happens

Trigger: Thrown at internal/js/modules/k6/browser/common/helpers.go:80 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/219e42a3fa31033c. Report an issue: GitHub.

Appendix: source

Thrown at internal/js/modules/k6/browser/common/helpers.go:80

		case math.Inf(0):
			unserVal = "Infinity"
		case math.Inf(-1):
			unserVal = "-Infinity"
		default:
			if math.IsNaN(a) {
				unserVal = "NaN"
			}
		}

		if unserVal != "" {
			return &cdpruntime.CallArgument{
				UnserializableValue: cdpruntime.UnserializableValue(unserVal),
			}, nil
		}

		b, err := json.Marshal(a)
		if err != nil {
			err = fmt.Errorf("converting argument '%v': %w", arg, err)
		}

		return &cdpruntime.CallArgument{Value: b}, err
	case *ElementHandle:
		return convertBaseJSHandleTypes(ctx, execCtx, &a.BaseJSHandle)
	case *BaseJSHandle:
		return convertBaseJSHandleTypes(ctx, execCtx, a)
	default:
		b, err := json.Marshal(a)
		return &cdpruntime.CallArgument{Value: b}, err //nolint:wrapcheck
	}
}

func call(
	ctx context.Context, fn func(context.Context, chan any, chan error), timeout time.Duration,
) (any, error) {
	var (
		result   any

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