diff --git a/ChangeLog b/ChangeLog
index ed984ca912..7b3353a50a 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+Tue Jun 11 23:01:57 2013  Tanaka Akira  <akr@fsij.org>
+
+	* bignum.c (validate_integer_pack_format): Refine error messages.
+
 Tue Jun 11 22:25:04 2013  Tanaka Akira  <akr@fsij.org>
 
 	* bignum.c (validate_integer_pack_format): numwords argument added.
diff --git a/bignum.c b/bignum.c
index 8e7e44427d..3eb086c66e 100644
--- a/bignum.c
+++ b/bignum.c
@@ -739,10 +739,16 @@ validate_integer_pack_format(size_t numwords, size_t wordsize, size_t nails, int
 {
     int wordorder_bits = flags & INTEGER_PACK_WORDORDER_MASK;
     int byteorder_bits = flags & INTEGER_PACK_BYTEORDER_MASK;
-    if (wordorder_bits != INTEGER_PACK_MSWORD_FIRST &&
+    if (wordorder_bits == 0) {
+        rb_raise(rb_eArgError, "word order not specified");
+    }
+    else if (wordorder_bits != INTEGER_PACK_MSWORD_FIRST &&
         wordorder_bits != INTEGER_PACK_LSWORD_FIRST)
         rb_raise(rb_eArgError, "unexpected word order");
-    if (byteorder_bits != INTEGER_PACK_MSBYTE_FIRST &&
+    if (byteorder_bits == 0) {
+        rb_raise(rb_eArgError, "byte order not specified");
+    }
+    else if (byteorder_bits != INTEGER_PACK_MSBYTE_FIRST &&
         byteorder_bits != INTEGER_PACK_LSBYTE_FIRST &&
         byteorder_bits != INTEGER_PACK_NATIVE_BYTE_ORDER)
         rb_raise(rb_eArgError, "unexpected byte order");