gohugoio/hugo · error

type %T is not a PageGroup

Error message

type %T is not a PageGroup

What it means

Returned by PageGroup.Slice when converting a []any into a PagesGroup for the template `slice`/collections functions: every element must be a PageGroup, and one element is some other type. Hugo fails fast rather than silently dropping the mismatched element.

Source

Thrown at resources/page/pagegroup.go:393

	if p.Key != otherP.Key {
		return false
	}

	return p.Pages.ProbablyEq(otherP.Pages)
}

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

// Len returns the number of pages in the page group.
func (psg PagesGroup) Len() int {
	l := 0
	for _, pg := range psg {
		l += len(pg.Pages)
	}
	return l
}

View on GitHub ↗ (pinned to 8a468df065)

Solutions

1. Ensure every element of the slice is a PageGroup — don't mix Pages, strings, or maps with grouped results
2. Use the output of .GroupBy* directly instead of rebuilding it with slice/append
3. If you need pages, use .Pages of each group rather than the groups themselves

Example fix

// before
{{ $mixed := slice (index $groups 0) .Site.RegularPages }}
// after
{{ $onlyGroups := slice (index $groups 0) (index $groups 1) }}

{{ $isPageGroup := and (reflect.IsMap . | not) (isset . "Key") (isset . "Pages") }}

When it happens

Trigger: Template code building a slice that mixes PageGroup values with other types, e.g. `{{ $g := slice (index $groups 0) .Site.Params.foo }}` or appending a Page/string into a slice whose first element is a PageGroup, then passing it where a PagesGroup is expected.

Common situations: Custom templates combining results of .GroupBy/.GroupByDate with plain pages or strings, iterating grouped output and rebuilding slices incorrectly, or partials that receive heterogeneous data.

Related errors

• arguments to symdiff must be slices or arrays
• can't iterate over %T
• text must be a string
• first argument must be a map
• type %T not supported

AI-assisted analysis of gohugoio/hugo@8a468df065 (2026-07-31). Data as JSON: /data/errors/34b2904ed9795158.json. Report an issue: GitHub ↗.