gohugoio/hugo · error

type %T is not a Resource

Error message

type %T is not a Resource

What it means

Returned by `commonResource.Slice` when iterating a `[]any` whose elements are not all `resource.Resource`. This is the resource-side adapter for `collections.Slice`, used to normalize heterogeneous template slices into `resource.Resources`. The per-element assertion at resource.go:290-292 fails on the first non-Resource element.

Source

Thrown at resources/resource.go:292

	baseResourceResource
	baseResourceInternal
	resource.Staler
}

type commonResource struct{}

// Slice is for internal use.
// for the template functions. See collections.Slice.
func (commonResource) Slice(in any) (any, error) {
	switch items := in.(type) {
	case resource.Resources:
		return items, nil
	case []any:
		groups := make(resource.Resources, len(items))
		for i, v := range items {
			g, ok := v.(resource.Resource)
			if !ok {
				return nil, fmt.Errorf("type %T is not a Resource", v)
			}
			groups[i] = g
			{
			}
		}
		return groups, nil
	default:
		return nil, fmt.Errorf("invalid slice type %T", items)
	}
}

type fileInfo interface {
	setOpenSource(hugio.OpenReadSeekCloser)
	setSourceFilenameIsHash(bool)
	setTargetPath(internal.ResourcePaths)
	size() int64
	hashProvider
}

View on GitHub (pinned to 52c9bd7908)

Solutions

1. Ensure every element of the input slice satisfies `resource.Resource` (filter out Pages/strings first).
2. Use `.Resources` directly or `resources.Get`/`resources.Match` to build the slice.
3. If you need to mix text, create a Resource first via `resources.FromString`.

Example fix

// template -- before
{{ $r := slice "main.js" $page.Resources.Get "app.js" }}

// after
{{ $r := slice (resources.Get "js/main.js") (resources.Get "js/app.js") }}

// Go
func isResource(v any) bool {
	_, ok := v.(resource.Resource)
	return ok
}
// Template
{{ $clean := slice }}
{{ range $r := $input }}{{ if (eq (printf "%T" $r) "*resources.resourceAdapter") }}{{ $clean = $clean | append $r }}{{ end }}{{ end }}

When it happens

Trigger: A template calls `collections.Slice` (or a pipe through `apply`) on a list containing non-resource values -- e.g. mixing `.Resources` entries with strings or pages, or passing page resources alongside plain Pages.

Common situations: Templates that build a JS/CSS bundle from `.Resources` plus a hardcoded string path; partial reuse where the param is sometimes a Resource and sometimes a Page; `slice` template function called with mixed media inputs.

Related errors

• invalid slice type %T
• type %T is not a WeightedPage
• invalid slice type %T
• %T cannot be merged by language
• called apply using {xt} as type {targ}

AI-assisted analysis of gohugoio/hugo@52c9bd7908 (2026-08-09). Data as JSON: /api/errors/4f8391e9b876bcc5. Report an issue: GitHub.