gohugoio/hugo · error

cannot 3-index slice a string

Error message

cannot 3-index slice a string

What it means

Raised by the `slice` action specifically when the item is a string and exactly three indexes are given (funcs.go:257-258). Go does not permit 3-index slice expressions on strings (`s[i:j:k]` is only valid for arrays/slices), so the template engine rejects it up front.

Source

Thrown at tpl/internal/go_templates/texttemplate/funcs.go:258

// is x[:], "slice x 1" is x[1:], and "slice x 1 2 3" is x[1:2:3]. The first
// argument must be a string, slice, or array.
func slice(item reflect.Value, indexes ...reflect.Value) (reflect.Value, error) {
	item = indirectInterface(item)
	if !item.IsValid() {
		return reflect.Value{}, fmt.Errorf("slice of untyped nil")
	}
	var isNil bool
	if item, isNil = indirect(item); isNil {
		return reflect.Value{}, fmt.Errorf("slice of nil pointer")
	}
	if len(indexes) > 3 {
		return reflect.Value{}, fmt.Errorf("too many slice indexes: %d", len(indexes))
	}
	var cap int
	switch item.Kind() {
	case reflect.String:
		if len(indexes) == 3 {
			return reflect.Value{}, fmt.Errorf("cannot 3-index slice a string")
		}
		cap = item.Len()
	case reflect.Array, reflect.Slice:
		cap = item.Cap()
	default:
		return reflect.Value{}, fmt.Errorf("can't slice item of type %s", item.Type())
	}
	// set default values for cases item[:], item[i:].
	idx := [3]int{0, item.Len()}
	for i, index := range indexes {
		x, err := indexArg(index, cap)
		if err != nil {
			return reflect.Value{}, err
		}
		idx[i] = x
	}
	// given item[i:j], make sure i <= j.
	if idx[0] > idx[1] {

View on GitHub (pinned to 52c9bd7908)

Solutions

1. Drop the third index for strings: `{{slice .Str 0 2}}`.
2. If you really need cap-bounded slicing, convert to `[]byte`/`[]rune` in Go first.

Example fix

// before
{{slice .Name 0 3 5}}   // .Name is string

// after
{{slice .Name 0 3}}

// For strings, use at most 2 indexes:
//   {{slice .Str 0 3}}
// If you need cap semantics, operate on []byte in Go first.

func isString(v interface{}) bool {
    return v != nil && reflect.TypeOf(v).Kind() == reflect.String
}

When it happens

Trigger: `{{slice .Str 0 2 4}}` where `.Str` is a string; trying to bound a string sub-slice's capacity like an array.

Common situations: Treating a string like a `[]byte`; helper template written for slices being reused on strings; front matter that changed from a byte slice to a string.

Related errors

• slice of untyped nil
• slice of nil pointer
• too many slice indexes: %d
• can't slice item of type %s
• invalid slice index: %d > %d

AI-assisted analysis of gohugoio/hugo@52c9bd7908 (2026-08-09). Data as JSON: /api/errors/d6b484bacb6ffa5d. Report an issue: GitHub.