diff --git a/ruby/extconf.rb b/ruby/extconf.rb
index 10a807e7..4082a92f 100644
--- a/ruby/extconf.rb
+++ b/ruby/extconf.rb
@@ -1,5 +1,6 @@
 require 'mkmf'
 require './version.rb'
 $CFLAGS << %[ -I.. -Wall -O3 -DMESSAGEPACK_VERSION=\\"#{MessagePack::VERSION}\\" -g]
+have_header("ruby/st.h")
 create_makefile('msgpack')
 
diff --git a/ruby/pack.c b/ruby/pack.c
index c0e135c8..c09bf0f1 100644
--- a/ruby/pack.c
+++ b/ruby/pack.c
@@ -15,6 +15,7 @@
  *    See the License for the specific language governing permissions and
  *    limitations under the License.
  */
+#define RSTRING_NOT_MODIFIED
 #include "ruby.h"
 #include "compat.h"
 
@@ -39,8 +40,8 @@ static ID s_append;
 #include "msgpack/pack_template.h"
 
 
-#ifndef RUBY_VM
-#include "st.h"  // ruby hash
+#ifdef HAVE_RUBY_ST_H
+#include "ruby/st.h"  // ruby hash
 #endif
 
 #define ARG_BUFFER(name, argc, argv) \
@@ -219,11 +220,12 @@ static VALUE MessagePack_Array_to_msgpack(int argc, VALUE *argv, VALUE self)
 {
 	ARG_BUFFER(out, argc, argv);
 	// FIXME check sizeof(long) > sizeof(unsigned int) && RARRAY_LEN(self) > UINT_MAX
-	msgpack_pack_array(out, (unsigned int)RARRAY_LEN(self));
-	VALUE* p = RARRAY_PTR(self);
-	VALUE* const pend = p + RARRAY_LEN(self);
-	for(;p != pend; ++p) {
-		rb_funcall(*p, s_to_msgpack, 1, out);
+	unsigned int ary_length = (unsigned int)RARRAY_LEN(self);
+	unsigned int i = 0;
+	msgpack_pack_array(out, ary_length);
+	for(; i < ary_length; ++i) {
+		VALUE p = rb_ary_entry(self, i);
+		rb_funcall(p, s_to_msgpack, 1, out);
 	}
 	return out;
 }
diff --git a/ruby/unpack.c b/ruby/unpack.c
index f61fe6db..a3bb861d 100644
--- a/ruby/unpack.c
+++ b/ruby/unpack.c
@@ -112,7 +112,7 @@ static inline int template_callback_array(unpack_user* u, unsigned int n, VALUE*
 { *o = rb_ary_new2(n); return 0; }
 
 static inline int template_callback_array_item(unpack_user* u, VALUE* c, VALUE o)
-{ rb_ary_push(*c, o); return 0; }  // FIXME set value directry RARRAY_PTR(obj)[RARRAY_LEN(obj)++]
+{ rb_ary_push(*c, o); return 0; }
 
 static inline int template_callback_map(unpack_user* u, unsigned int n, VALUE* o)
 { *o = rb_hash_new(); return 0; }