grafana/k6 · error · TypeError

invalid value for metric metadata '%s': only String, Boolean

Error message

invalid value for metric metadata '%s': only String, Boolean and Number types are accepted as a metric metadata value

What it means

metadataDynamicObject.Set (backing execution.vu.metadata[key]=value) received a value that cannot be stored as metric metadata: only String, Boolean and Number types are accepted. Any other type (object, null, undefined, symbol, function) triggers an exception in the JS runtime at assignment time.

Source

Thrown at internal/js/modules/k6/execution/execution.go:431

	state   *lib.State
}

// Get a property value for the key. May return nil if the property does not exist.
func (o *metadataDynamicObject) Get(key string) sobek.Value {
	tcv := o.state.Tags.GetCurrentValues()
	if metadatum, ok := tcv.Metadata[key]; ok {
		return o.runtime.ToValue(metadatum)
	}
	return nil
}

// Set a property value for the key. It returns true if successful. String, Boolean
// and Number types are implicitly converted to the Sobek's relative string
// representation. An exception is raised in case a denied type is provided.
func (o *metadataDynamicObject) Set(key string, val sobek.Value) bool {
	o.state.Tags.Modify(func(tagsAndMeta *metrics.TagsAndMeta) {
		if err := common.ApplyCustomUserMetadata(tagsAndMeta, key, val); err != nil {
			panic(o.runtime.NewTypeError(err.Error()))
		}
	})
	return true
}

// Has returns true if the property exists.
func (o *metadataDynamicObject) Has(key string) bool {
	ctv := o.state.Tags.GetCurrentValues()
	if _, ok := ctv.Metadata[key]; ok {
		return true
	}
	return false
}

// Delete deletes the property for the key. It returns true on success (note,
// that includes missing property).
func (o *metadataDynamicObject) Delete(key string) bool {
	o.state.Tags.Modify(func(tagsAndMeta *metrics.TagsAndMeta) {

View on GitHub (pinned to 01ffac6f24)

Solutions

1. Assign only strings, booleans, or numbers to execution.vu.metadata entries
2. Stringify complex values before assignment (e.g. String(obj) or JSON.stringify)
3. Avoid null/undefined metadata values

When it happens

Trigger: Thrown at internal/js/modules/k6/execution/execution.go:431 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/382baa873c2aa20d. Report an issue: GitHub.