diff --git a/smalltrip/middleware/cache.go b/smalltrip/middleware/cache.go
index 7d1154a..c2fb4b5 100644
--- a/smalltrip/middleware/cache.go
+++ b/smalltrip/middleware/cache.go
@@ -142,6 +142,8 @@ type directives struct {
 	staleIfError         *time.Duration
 }
 
+var _ fmt.Stringer = directives{}
+
 func (d directives) String() string {
 	ds := []string{}