What causes "exec value cannot be empty" in grafana/k6?

Remove the exec property from the scenario so it falls back to the default exported function

exec value cannot be empty

Error message

exec value cannot be empty

What it means

Emitted by BaseConfig.Validate (lib/executor/base_config.go:59-61) when the scenario's exec field is present and Valid but set to an empty string. exec is a null.String: it is only meant to hold the name of an exported JS function that this scenario should run instead of default. An explicitly empty string is treated as a configuration mistake — omitting the field entirely is the correct way to use the default function.

Source

Thrown at lib/executor/base_config.go:60

	return BaseConfig{
		Name:         name,
		Type:         configType,
		GracefulStop: types.NewNullDuration(DefaultGracefulStopValue, false),
	}
}

// Validate checks some basic things like present name, type, and a positive start time
func (bc BaseConfig) Validate() (result []error) {
	// Some just-in-case checks, since those things are likely checked in other places or
	// even assigned by us:
	if bc.Name == "" {
		result = append(result, errors.New("scenario name can't be empty"))
	}
	if !scenarioNameWhitelist.MatchString(bc.Name) {
		result = append(result, errors.New(scenarioNameErr))
	}
	if bc.Exec.Valid && bc.Exec.String == "" {
		result = append(result, errors.New("exec value cannot be empty"))
	}
	if bc.Type == "" {
		result = append(result, errors.New("missing or empty type field"))
	}
	// The actually reasonable checks:
	if bc.StartTime.Duration < 0 {
		result = append(result, errors.New("the startTime can't be negative"))
	}
	if bc.GracefulStop.Duration < 0 {
		result = append(result, errors.New("the gracefulStop timeout can't be negative"))
	}
	return result
}

// GetName returns the name of the scenario.
func (bc BaseConfig) GetName() string {
	return bc.Name
}

View on GitHub (pinned to 93accf6570)

Solutions

Remove the exec property from the scenario so it falls back to the default exported function
Or set it to an actual exported function name defined in the script, e.g. exec: 'login'
If the value comes from an env var, default it or conditionally include the property only when non-empty

Example fix

// before
export const options = {
  scenarios: {
    smoke: { executor: 'shared-iterations', exec: '' },
  },
};

// after
export const options = {
  scenarios: {
    smoke: { executor: 'shared-iterations' }, // uses default export
  },
};

Defensive patterns

Strategy: validation

Validate before calling

// Only include exec when it names a real exported function
const fn = __ENV.SCENARIO_FUNC;
export const options = {
  scenarios: {
    smoke: {
      executor: 'shared-iterations',
      ...(fn ? { exec: fn } : {}),
    },
  },
};

Prevention

Never write exec: '' — omit the key to use the default export
When exec is env-driven, default it and assert the function exists: if (typeof globalThis[fn] !== 'function') throw new Error(...)
Remember k6 only accepts exported top-level functions as exec targets

When it happens

Trigger: Setting scenarios: { smoke: { executor: 'per-vu-iterations', exec: '' } } in exported options, or unmarshaling scenario JSON that contains an "exec": "" member. Validate() fires during config validation before the test starts.

Common situations: Templated scripts that interpolate a function name from a variable which is empty (e.g. __ENV.SCENARIO_FUNC unset), or a leftover empty exec after commenting out the intended function name.

Related errors

AI-assisted analysis of grafana/k6@93accf6570 (2026-08-15). Data as JSON: /api/errors/7b363a8a945596c1. Report an issue: GitHub.