gohugoio/hugo · error

type %T not supported

Error message

type %T not supported

What it means

`transform.Unmarshal` received a value that is neither an UnmarshableResource nor convertible to a string (via `types.ToStringE`). The function only accepts Resource objects or raw string data; passing maps, slices, Pages, or already-decoded data lands here, with the Go type printed via %T.

Source

Thrown at tpl/transform/unmarshal.go:121

			return &resources.StaleValue[any]{
				Value: v,
				StaleVersionFunc: func() uint32 {
					return resource.StaleVersion(r)
				},
			}, nil
		})
		if err != nil {
			return nil, err
		}

		return v.Value, nil

	}

	dataStr, err := types.ToStringE(data)
	if err != nil {
		return nil, fmt.Errorf("type %T not supported", data)
	}

	if strings.TrimSpace(dataStr) == "" {
		return nil, nil
	}

	key := hashing.XxHashFromStringHexEncoded(dataStr)

	if decoder != metadecoders.Default {
		key += decoder.OptionsKey()
	}

	v, err := ns.cacheUnmarshal.GetOrCreate(key, func(string) (*resources.StaleValue[any], error) {
		var f metadecoders.Format
		if decoder.Format != "" {
			f = metadecoders.FormatFromString(decoder.Format)
			if f == "" {
				return nil, fmt.Errorf("format %q not supported", decoder.Format)

View on GitHub ↗ (pinned to 8a468df065)

Solutions

1. Pass raw string content or a Resource: e.g. `transform.Unmarshal ($file.Content)` or `transform.Unmarshal $resource`.
2. Remove redundant unmarshal calls — data from `site.Data` or a prior `unmarshal` is already decoded.
3. When using options, check argument order: `transform.Unmarshal OPTIONS DATA` (options map first).

Example fix

{{/* before */}}
{{ $d := site.Data.config | transform.Unmarshal }}
{{/* after */}}
{{ $d := site.Data.config }}{{/* already decoded */}}

{{/* Unmarshal accepts only strings, []byte, or Resources */}}
{{ if or (reflect.IsMap . | not) (eq (printf "%T" .) "string") }}...{{ end }}
{{/* simplest: coerce first */}}
{{ $data := transform.Unmarshal (string $input) }}

{{ with try (transform.Unmarshal $input) }}
  {{ with .Err }}{{ errorf "cannot unmarshal %T: %s" $input . }}{{ else }}{{ $data := .Value }}{{ end }}
{{ end }}

When it happens

Trigger: Calling `transform.Unmarshal` with a non-string, non-Resource argument — e.g. a map/slice, a Page object, nil-ish complex values, or piping already-unmarshaled data through `unmarshal` a second time.

Common situations: Double-unmarshaling (`.Content | unmarshal | unmarshal`), passing `.Params` or a data object from `site.Data` (already decoded) instead of raw text, or getting arguments in the wrong order in a dict-options call so the options map is treated as data.

Related errors

• first argument must be a map
• arguments to symdiff must be slices or arrays
• can't iterate over %T
• text must be a string
• unmarshal takes 1 or 2 arguments

AI-assisted analysis of gohugoio/hugo@8a468df065 (2026-07-31). Data as JSON: /data/errors/44b2b70b0be72bb3.json. Report an issue: GitHub ↗.