diff --git a/ext/json/lib/json/add/core.rb b/ext/json/lib/json/add/core.rb
index 4423e7ad75..7121a77ff1 100644
--- a/ext/json/lib/json/add/core.rb
+++ b/ext/json/lib/json/add/core.rb
@@ -96,7 +96,7 @@ class Struct
 
   def to_json(*args)
     klass = self.class.name
-    klass.to_s.empty? and raise JSON::JSONError, "Only named structs are supported!"
+    klass.nil? and raise JSON::JSONError, "Only named structs are supported!"
     {
       'json_class' => klass,
       'v'     => values,