What causes "need at least 2 arguments to append" in gohugoio/hugo?

Pass at least one element plus the destination slice: {{ $pages | append .Page }}

need at least 2 arguments to append

Error message

need at least 2 arguments to append

What it means

Append (append.go:31) takes variadic args where the LAST arg is the destination slice and the rest are elements to add. With fewer than 2 args there is no element+slice pair, so the call is rejected at line 33 before any reflection. The message is an explicit arity guard.

Solutions

Pass at least one element plus the destination slice: {{ $pages | append .Page }}
To merge two slices of the same element type, pass both: {{ append $morePages $pages }}
When piping, always provide the element(s) after the pipe target

Example fix

// before
{{ $pages | append }}
// after
{{ $pages | append .Page }}

Defensive patterns

Strategy: validation

Validate before calling

{{/* ensure at least one element plus the slice before appending */}}
{{ if and $elem $pages }}
  {{ $pages = $pages | append $elem }}
{{ end }}

Prevention

Always pass the destination slice as the last positional arg and at least one element before it
When piping, remember the pipe target is the final arg, so you still need the element(s)
Treat append as needing (element..., slice) not (slice) alone

When it happens

Trigger: Calling {{ append .Pages }} with one arg, or {{ $pages | append }} where the pipe supplies the only argument (len(args) < 2). The pipe target becomes the sole positional arg.

Common situations: Refactoring a template and dropping the appended value; piping a slice into append but forgetting to pass the element to add; calling append intending to clone a single slice.

Related errors

AI-assisted analysis of gohugoio/hugo@52c9bd7908 (2026-08-09). Data as JSON: /api/errors/41fe87fc4e5117ee. Report an issue: GitHub.

Appendix: source

Thrown at tpl/collections/append.go:33

import (
	"errors"

	"github.com/gohugoio/hugo/common/collections"
)

// Append appends args up to the last one to the slice in the last argument.
// This construct allows template constructs like this:
//
//	{{ $pages = $pages | append $p2 $p1 }}
//
// Note that with 2 arguments where both are slices of the same type,
// the first slice will be appended to the second:
//
//	{{ $pages = $pages | append .Site.RegularPages }}
func (ns *Namespace) Append(args ...any) (any, error) {
	if len(args) < 2 {
		return nil, errors.New("need at least 2 arguments to append")
	}

	to := args[len(args)-1]
	from := args[:len(args)-1]

	return collections.Append(to, from...)
}

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