NullObject#as_json returns ‘null’

lib/twitter/null_object.rb

@@ -41,5 +41,9 @@ def <=>(other)
     def nil?
       true
     end
+
+    def as_json
+      'null'
+    end
   end
 end

spec/twitter/null_object_spec.rb

@@ -55,6 +55,12 @@
     end
   end
 
+  describe '#as_json' do
+    it "returns 'null'" do
+      expect(subject.as_json).to eq('null')
+    end
+  end
+
   describe 'black hole' do
     it 'returns self for missing methods' do
       expect(subject.missing).to eq(subject)