diff --git a/ChangeLog b/ChangeLog
index 845f2829f4..86a9956f6d 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+Sat Dec 11 09:24:57 2010  Tanaka Akira  <akr@fsij.org>
+
+	* encoding.c: parenthesize macro arguments.
+
 Sat Dec 11 08:12:48 2010  Eric Hodel  <drbrain@segment7.net>
 
 	* ext/openssl/ossl.c, ext/openssl/ossl_pkey_rsa.c: Document RSA, RSA
diff --git a/encoding.c b/encoding.c
index ac0e1ba842..2a9881bb08 100644
--- a/encoding.c
+++ b/encoding.c
@@ -1556,7 +1556,7 @@ Init_Encoding(void)
 /* locale insensitive ctype functions */
 
 #define ctype_test(c, ctype) \
-    (rb_isascii(c) && ONIGENC_IS_ASCII_CODE_CTYPE((c), ctype))
+    (rb_isascii(c) && ONIGENC_IS_ASCII_CODE_CTYPE((c), (ctype)))
 
 int rb_isalnum(int c) { return ctype_test(c, ONIGENC_CTYPE_ALNUM); }
 int rb_isalpha(int c) { return ctype_test(c, ONIGENC_CTYPE_ALPHA); }