gohugoio/hugo · error

len of nil pointer

Error message

len of nil pointer

What it means

Raised by the `len` template function when its argument, after `indirect`, is a nil pointer (funcs.go:293-295). `len` cannot measure something that does not exist; a nil `*[]T`/`*string`/`*map` has no length.

Source

Thrown at tpl/internal/go_templates/texttemplate/funcs.go:295

		return reflect.Value{}, fmt.Errorf("invalid slice index: %d > %d", idx[0], idx[1])
	}
	if len(indexes) < 3 {
		return item.Slice(idx[0], idx[1]), nil
	}
	// given item[i:j:k], make sure i <= j <= k.
	if idx[1] > idx[2] {
		return reflect.Value{}, fmt.Errorf("invalid slice index: %d > %d", idx[1], idx[2])
	}
	return item.Slice3(idx[0], idx[1], idx[2]), nil
}

// Length

// length returns the length of the item, with an error if it has no defined length.
func length(item reflect.Value) (int, error) {
	item, isNil := indirect(item)
	if isNil {
		return 0, fmt.Errorf("len of nil pointer")
	}
	switch item.Kind() {
	case reflect.Array, reflect.Chan, reflect.Map, reflect.Slice, reflect.String:
		return item.Len(), nil
	}
	return 0, fmt.Errorf("len of type %s", item.Type())
}

// Function invocation

func emptyCall(fn reflect.Value, args ...reflect.Value) reflect.Value {
	panic("unreachable") // implemented as a special case in evalCall
}

// call returns the result of evaluating the first argument as a function.
// The function must return 1 result, or 2 results, the second of which is an error.
func call(name string, fn reflect.Value, args ...reflect.Value) (reflect.Value, error) {
	fn = indirectInterface(fn)

View on GitHub (pinned to 52c9bd7908)

Solutions

1. Guard with `{{with .Ptr}}{{len .}}{{else}}0{{end}}`.
2. Change the field to a value type (`[]T` instead of `*[]T`) or initialize it.
3. Provide a non-nil default in the data layer.

Example fix

// before
{{len .Items}}   // .Items is *[]string, nil

// after
{{with .Items}}{{len .}}{{else}}0{{end}}

// Guard optional pointer fields:
//   {{with .Ptr}}{{len .}}{{else}}0{{end}}

func nonNilPointerOrZero(v interface{}) int {
    rv := reflect.ValueOf(v)
    if !rv.IsValid() { return 0 }
    if rv.Kind() == reflect.Ptr && rv.IsNil() { return 0 }
    if rv.Kind() == reflect.Ptr { rv = rv.Elem() }
    switch rv.Kind() {
    case reflect.Array, reflect.Chan, reflect.Map, reflect.Slice, reflect.String:
        return rv.Len()
    }
    return 0
}

When it happens

Trigger: `{{len .Ptr}}` where `.Ptr` is a nil pointer to a slice/string/map/chan/array; optional fields typed as pointers that were never set.

Common situations: Optional front matter or struct fields exposed as pointers; JSON unmarshaling that leaves absent fields nil; migrations from value types to pointer types.

Related errors

• slice of nil pointer
• len of type %s
• can't iterate over a nil value
• can't iterate over a nil value of type {type}
• index of nil pointer

AI-assisted analysis of gohugoio/hugo@52c9bd7908 (2026-08-09). Data as JSON: /api/errors/270db04a4d614024. Report an issue: GitHub.