gohugoio/hugo · error

language merge not supported for %T

Error message

language merge not supported for %T

What it means

`lang.Merge` unions page collections across languages by requiring the first collection (p1) to implement Hugo's internal `pagesLanguageMerger` interface (`MergeByLanguageInterface`). Only Hugo Pages collections implement it; if p1 is any other type (a plain slice, map, string, etc.), Hugo returns this error with the offending type.

Source

Thrown at tpl/lang/lang.go:246

	return string(b), nil
}

type pagesLanguageMerger interface {
	MergeByLanguageInterface(other any) (any, error)
}

// Merge creates a union of pages from two languages.
func (ns *Namespace) Merge(p2, p1 any) (any, error) {
	if !hreflect.IsTruthful(p1) {
		return p2, nil
	}
	if !hreflect.IsTruthful(p2) {
		return p1, nil
	}
	merger, ok := p1.(pagesLanguageMerger)
	if !ok {
		return nil, fmt.Errorf("language merge not supported for %T", p1)
	}
	return merger.MergeByLanguageInterface(p2)
}

View on GitHub ↗ (pinned to 8a468df065)

Solutions

1. Ensure both arguments are Pages collections, e.g. `{{ $merged := site.RegularPages | lang.Merge ((index site.Sites 1).RegularPages) }}`.
2. If filtering first, use `where` directly on a Pages collection (which preserves the Pages type) rather than building a new slice with `slice`/`append` of mixed values.
3. For non-page data, use `collections.Merge` or `append`/`union` instead — `lang.Merge` is only for pages across languages.

Example fix

<!-- before -->
{{ $merged := (slice $p1 $p2) | lang.Merge $frPages }}
<!-- after -->
{{ $merged := site.RegularPages | lang.Merge $frSite.RegularPages }}

{{ $v := site.Params.someMap }}
{{ if reflect.IsMap $v }}{{/* safe to merge language params */}}{{ end }}

{{ $isMergeable := reflect.IsMap $v }}

{{ with try (merge $base $override) }}{{ with .Err }}{{ warnf "language merge failed: %s" . }}{{ else }}{{ .Value }}{{ end }}{{ end }}

When it happens

Trigger: Calling `{{ $pages | lang.Merge $other }}` where the second (pipeline) argument is not a Hugo Pages collection — e.g. a `slice`, the result of `where` that produced a generic slice, a map, or a single Page. Note both arguments are checked for truthiness first, so an empty p1 skips the check.

Common situations: Multilingual sites where a template passes `.Site.Data` values, a plain `slice`, or a single `.Page` instead of `.Site.RegularPages`/`.Translations`-derived page collections; applying `lang.Merge` to results of `collections` functions that returned `[]interface{}` instead of Pages.

Related errors

• grouping not supported for type %T %T
• %T cannot be merged by language
• cannot convert type %T to Pages
• error building site: %w
• multihost change detected, please restart server

AI-assisted analysis of gohugoio/hugo@8a468df065 (2026-07-31). Data as JSON: /data/errors/8781e61cdd7ae5a5.json. Report an issue: GitHub ↗.