diff --git a/lib/json/truffle_ruby/generator.rb b/lib/json/truffle_ruby/generator.rb
index 13b15459..09459286 100644
--- a/lib/json/truffle_ruby/generator.rb
+++ b/lib/json/truffle_ruby/generator.rb
@@ -517,7 +517,7 @@ def json_transform(state)
 
             if empty?
               state.depth -= 1
-              return '{}'
+              return +'{}'
             end
 
             delim = ",#{state.object_nl}"
@@ -609,7 +609,7 @@ def json_transform(state)
 
             if empty?
               state.depth -= 1
-              return '[]'
+              return +'[]'
             end
 
             result = '['.dup
@@ -734,17 +734,17 @@ def to_json(state = nil, *args)
 
         module TrueClass
           # Returns a JSON string for true: 'true'.
-          def to_json(*) 'true' end
+          def to_json(*) +'true' end
         end
 
         module FalseClass
           # Returns a JSON string for false: 'false'.
-          def to_json(*) 'false' end
+          def to_json(*) +'false' end
         end
 
         module NilClass
           # Returns a JSON string for nil: 'null'.
-          def to_json(*) 'null' end
+          def to_json(*) +'null' end
         end
       end
     end
diff --git a/test/json/json_generator_test.rb b/test/json/json_generator_test.rb
index eb5a8ca5..1e44117e 100755
--- a/test/json/json_generator_test.rb
+++ b/test/json/json_generator_test.rb
@@ -84,6 +84,24 @@ def test_dump_strict
     assert_equal '"World"', "World".to_json(strict: true)
   end
 
+  def test_not_frozen
+    [
+      [[], '[]'],
+      [{}, '{}'],
+      ["string", '"string"'],
+      [:sym, '"sym"'],
+      [1, '1'],
+      [1.0, '1.0'],
+      [true, 'true'],
+      [false, 'false'],
+      [nil, 'null'],
+    ].each do |(obj, exp)|
+      dumped = dump(obj, strict: true)
+      assert_equal exp, dumped
+      refute_predicate dumped, :frozen?
+    end
+  end
+
   def test_state_depth_to_json
     depth = Object.new
     def depth.to_json(state)