diff --git a/lib/json/truffle_ruby/generator.rb b/lib/json/truffle_ruby/generator.rb
index 01911645..d0ed0826 100644
--- a/lib/json/truffle_ruby/generator.rb
+++ b/lib/json/truffle_ruby/generator.rb
@@ -349,7 +349,7 @@ def generate(obj, anIO = nil)
               !@ascii_only and !@script_safe and @max_nesting == 0 and (!@strict || Symbol === obj)
             result = generate_json(obj, ''.dup)
           else
-            result = obj.to_json(self)
+            result = +obj.to_json(self)
           end
           JSON::TruffleRuby::Generator.valid_utf8?(result) or raise GeneratorError.new(
             "source sequence #{result.inspect} is illegal/malformed utf-8",
@@ -517,11 +517,11 @@ def json_transform(state)
 
             if empty?
               state.depth -= 1
-              return +'{}'
+              return '{}'
             end
 
             delim = ",#{state.object_nl}"
-            result = +"{#{state.object_nl}"
+            result = "{#{state.object_nl}"
             first = true
             key_type = nil
             indent = !state.object_nl.empty?
@@ -558,7 +558,7 @@ def json_transform(state)
                 raise TypeError, "#{key.class}#to_s returns an instance of #{key_str.class}, expected a String"
               end
 
-              result = +"#{result}#{key_json}#{state.space_before}:#{state.space}"
+              result = "#{result}#{key_json}#{state.space_before}:#{state.space}"
               if state.strict? && !Generator.native_type?(value)
                 if state.as_json
                   value = state.as_json.call(value, false)
@@ -609,7 +609,7 @@ def json_transform(state)
 
             if empty?
               state.depth -= 1
-              return +'[]'
+              return '[]'
             end
 
             result = '['.dup
@@ -736,17 +736,17 @@ 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