What causes "%s is a method of type %s but returns no output" in gohugoio/hugo?

Filter on a real value: a field or a method that returns the post-reset state.

%s is a method of type %s but returns no output

Error message

%s is a method of type %s but returns no output

What it means

`evaluateSubElem` (where.go:344-345) requires a method used as a `where` key to produce a value it can compare against the match argument. If `mt.Type.NumOut() == 0` the method returns nothing, so there is no value to filter on, and Hugo rejects it.

Source

Thrown at tpl/collections/where.go:345

	mt := hreflect.GetMethodByNameForType(objPtr.Type(), elemName)
	if mt.Func.IsValid() {
		// Receiver is the first argument.
		args := []reflect.Value{objPtr}
		num := mt.Type.NumIn()
		maxNumIn := 1
		if num > 1 && hreflect.IsContextType(mt.Type.In(1)) {
			args = append(args, ctx)
			maxNumIn = 2
		}

		switch {
		case mt.PkgPath != "":
			return zero, fmt.Errorf("%s is an unexported method of type %s", elemName, typ)
		case mt.Type.NumIn() > maxNumIn:
			return zero, fmt.Errorf("%s is a method of type %s but requires more than %d parameter", elemName, typ, maxNumIn)
		case mt.Type.NumOut() == 0:
			return zero, fmt.Errorf("%s is a method of type %s but returns no output", elemName, typ)
		case mt.Type.NumOut() > 2:
			return zero, fmt.Errorf("%s is a method of type %s but returns more than 2 outputs", elemName, typ)
		case mt.Type.NumOut() == 1 && mt.Type.Out(0).Implements(errorType):
			return zero, fmt.Errorf("%s is a method of type %s but only returns an error type", elemName, typ)
		case mt.Type.NumOut() == 2 && !mt.Type.Out(1).Implements(errorType):
			return zero, fmt.Errorf("%s is a method of type %s returning two values but the second value is not an error type", elemName, typ)
		}
		res := mt.Func.Call(args)
		if len(res) == 2 && !res[1].IsNil() {
			return zero, fmt.Errorf("error at calling a method %s of type %s: %s", elemName, typ, res[1].Interface().(error))
		}
		return res[0], nil
	}

	// elemName isn't a method so next start to check whether it is
	// a struct field or a map value. In both cases, it mustn't be
	// a nil value
	if isNil {

View on GitHub (pinned to 52c9bd7908)

Solutions

Filter on a real value: a field or a method that returns the post-reset state.
Expose an exported field carrying the boolean/value you actually want to match.
If you need the side effect, perform it in a `range` before filtering, not as a where key.

Example fix

// before
{{ where $items "Reset" "==" true }}  {{/* Reset() returns nothing */}}
// after
{{ where $items "NeedsReset" "==" true }}  {{/* NeedsReset() bool */}}

Defensive patterns

Strategy: validation

Validate before calling

// Go-side: only allow methods that return at least one value
mt, ok := reflect.TypeOf(item).MethodByName(name)
ok = ok && mt.Type.NumOut() >= 1

Prevention

Reserve `where` keys for value-returning query methods, not mutators.
Name command-style methods (Reset, Save, Load) distinctly from query methods.
Document which methods are template-safe.

When it happens

Trigger: `{{ where $items "Reset" "==" true }}` against a method `func (i *Item) Reset()` that performs a side effect with no return value.

Common situations: Treating a command/mutator method as a query; copy-pasting a method name that happens to be a void action; migrating from a field to a setter method.

Related errors

AI-assisted analysis of gohugoio/hugo@52c9bd7908 (2026-08-09). Data as JSON: /api/errors/aefb8536d1a39174. Report an issue: GitHub.