diff --git a/defaults.go b/defaults.go
index a768e019..4b1de62d 100644
--- a/defaults.go
+++ b/defaults.go
@@ -303,4 +303,5 @@ func (c Class) String() string {
 	} else {
 		return "CLASS" + strconv.Itoa(int(c))
 	}
+	panic("dns: not reached")
 }