gohugoio/hugo · error

type %T not supported in Resource transformations

Error message

type %T not supported in Resource transformations

What it means

A single argument was passed to a resource transformation but it does not implement resources.ResourceTransformer — the interface all pipeable Hugo assets satisfy. The %T verb reports the actual concrete type so the author can see what was piped in instead of a resource. It also fires in the two-arg path when args[1] is a plain map (options in the resource slot).

Source

Thrown at tpl/internal/resourcehelpers/helpers.go:50

	v1, ok1 := args[0].(string)
	if !ok1 {
		return nil, "", false
	}
	v2, ok2 := args[1].(resources.ResourceTransformer)

	return v2, v1, ok2
}

// This roundabout way of doing it is needed to get both pipeline behavior and options as arguments.
func ResolveArgs(args []any) (resources.ResourceTransformer, map[string]any, error) {
	if len(args) == 0 {
		return nil, nil, errors.New("no Resource provided in transformation")
	}

	if len(args) == 1 {
		r, ok := args[0].(resources.ResourceTransformer)
		if !ok {
			return nil, nil, fmt.Errorf("type %T not supported in Resource transformations", args[0])
		}
		return r, nil, nil
	}

	r, ok := args[1].(resources.ResourceTransformer)
	if !ok {
		if _, ok := args[1].(map[string]any); !ok {
			return nil, nil, fmt.Errorf("no Resource provided in transformation")
		}
		return nil, nil, fmt.Errorf("type %T not supported in Resource transformations", args[0])
	}

	m, err := hmaps.ToStringMapE(args[0])
	if err != nil {
		return nil, nil, fmt.Errorf("invalid options type: %w", err)
	}

	return r, m, nil

View on GitHub ↗ (pinned to 8a468df065)

Solutions

1. Fetch a real resource first: `{{ $r := resources.Get "css/main.css" }}{{ $r | minify }}`.
2. Check argument order for two-arg transforms: options first, resource last (`resources.PostCSS $opts $r` or `$r | resources.PostCSS $opts`).
3. Print the offending type with `printf "%T"` to see what you actually passed.

Example fix

<!-- before -->
{{ $min := "css/main.css" | minify }}
<!-- after -->
{{ $min := resources.Get "css/main.css" | minify }}

{{ $r := resources.Get "data.json" }}
{{ if $r }}{{ $min := $r | minify }}{{ end }}

{{/* ensure the value is a Resource, not a string path or Page */}}
{{ if reflect.IsMap $r }}{{ errorf "expected a Resource, got a map" }}{{ end }}

When it happens

Trigger: `{{ "css/main.css" | minify }}` (a string, not a resource); piping a Page, nil, or plain map into fingerprint/minify/toCSS; passing options where the resource should be.

Common situations: Confusing a file path string with a resource (must call resources.Get first); piping the result of readFile (a string) into a transform; page resources fetched with the wrong method returning a different type.

Related errors

• no Resource provided in transformation
• invalid options type: %w
• arguments to complement must be slices or arrays
• no Key set in Resource
• resources.PostProcess cannot be used in a deferred template

AI-assisted analysis of gohugoio/hugo@8a468df065 (2026-07-31). Data as JSON: /data/errors/104d69fb1a6987aa.json. Report an issue: GitHub ↗.