gohugoio/hugo · error

failed to convert %T to a string slice

Error message

failed to convert %T to a string slice

What it means

Returned by types.ToStringSlicePreserveStringE when cast.ToStringSliceE fails and the reflect fallback also fails (value is not a string, slice, or array). The fallback iterates slice/array elements but any other Kind lands in the default branch and yields this error naming the Go type.

Solutions

1. Supply a []string, []any, array, or a single string.
2. If a scalar is intended, wrap it in []string{fmt.Sprint(v)}.
3. Validate Kind==Slice/Array/String before calling.

Example fix

// before
types.ToStringSlicePreserveStringE(42)
// after
types.ToStringSlicePreserveStringE([]string{"42"})

func isStringSliceable(v any) bool {
  if v == nil { return true }
  switch reflect.ValueOf(v).Kind() {
  case reflect.String, reflect.Slice, reflect.Array: return true
  }
  return false
}

func isStringSliceable(v any) bool { k := reflect.ValueOf(v).Kind(); return k==reflect.String||k==reflect.Slice||k==reflect.Array }

When it happens

Trigger: Calling ToStringSlicePreserveStringE with a struct, map, number, bool, or pointer to non-slice. nil returns nil,nil; strings are wrapped; slices/arrays are element-wise stringified.

Common situations: Config fields expected to be lists but supplied as scalars (e.g. `menu.main.weight` instead of a list), or frontmatter where a YAML flow scalar replaces a sequence.

Related errors

• cannot convert to time.Duration
• --appendPort=false not supported when in multihost mode
• cache dir contains unresolved placeholders
• cannot determine BaseURL for protocol
• cannot unmarshal CSV into %T: invalid targetType: expected…

AI-assisted analysis of gohugoio/hugo@52c9bd7908 (2026-08-09). Data as JSON: /api/errors/9a7654913c9d6220. Report an issue: GitHub.

Appendix: source

Thrown at common/types/convert.go:81

		return result, nil
	}

	// Probably []int or similar. Fall back to reflect.
	vv := reflect.ValueOf(v)

	switch vv.Kind() {
	case reflect.Slice, reflect.Array:
		result = make([]string, vv.Len())
		for i := range vv.Len() {
			s, err := cast.ToStringE(vv.Index(i).Interface())
			if err != nil {
				return nil, err
			}
			result[i] = s
		}
		return result, nil
	default:
		return nil, fmt.Errorf("failed to convert %T to a string slice", v)
	}
}

// TypeToString converts v to a string if it's a valid string type.
// Note that this will not try to convert numeric values etc.,
// use ToString for that.
func TypeToString(v any) (string, bool) {
	switch s := v.(type) {
	case string:
		return s, true
	case template.HTML:
		return string(s), true
	case template.CSS:
		return string(s), true
	case template.HTMLAttr:
		return string(s), true
	case template.JS:
		return string(s), true

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