diff --git a/ext/extconf.rb b/ext/extconf.rb
index 7075478..aa72f54 100644
--- a/ext/extconf.rb
+++ b/ext/extconf.rb
@@ -68,6 +68,8 @@ def copy_gem(gem_dir)
 # thanks to: https://gist.github.com/IanVaughan/5489431
 $CPPFLAGS += " -DRUBY_19" if RUBY_VERSION =~ /1.9/
 $CPPFLAGS += " -DRUBY_20" if RUBY_VERSION =~ /2.0/
+$CPPFLAGS += " -DRUBY_21" if RUBY_VERSION =~ /2.1/
+$CPPFLAGS += " -DRUBY_22" if RUBY_VERSION =~ /2.2/
 
 puts "*** Using Ruby version: #{RUBY_VERSION}"
 puts "*** with CPPFLAGS: #{$CPPFLAGS}"
diff --git a/ext/native_server.c b/ext/native_server.c
index 11aee49..769524f 100644
--- a/ext/native_server.c
+++ b/ext/native_server.c
@@ -153,7 +153,7 @@ static VALUE mc_initialize(VALUE self, VALUE opts) {
   return self;
 }
 
-#if defined(RUBY_19) || defined(RUBY_20)
+#if defined(RUBY_19) || defined(RUBY_20) || defined(RUBY_21) || defined(RUBY_22)
 #define RSTRING_SET_LEN(str, newlen) (rb_str_set_len(str, new_len))
 #else
 #define RSTRING_SET_LEN(str, newlen) (RSTRING(str)->len = new_len)