diff --git a/ChangeLog b/ChangeLog
index 6788ce3f26..07d5c672e6 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Thu Feb 28 22:19:14 2008  NARUSE, Yui  <naruse@ruby-lang.org>
+
+	* encoding.c (enc_capable): IMMEDIATE_P doesn't include Qnil and Qfalse.
+	  use SPECIAL_CONST_P.
+
 Thu Feb 28 19:45:52 2008  NARUSE, Yui  <naruse@ruby-lang.org>
 
 	* encoding.c (enc_find): check type of argument and convert to String
diff --git a/encoding.c b/encoding.c
index 6cf32a81df..5c24c98505 100644
--- a/encoding.c
+++ b/encoding.c
@@ -535,7 +535,7 @@ rb_enc_find(const char *name)
 static inline int
 enc_capable(VALUE obj)
 {
-    if (IMMEDIATE_P(obj)) return Qfalse;
+    if (SPECIAL_CONST_P(obj)) return Qfalse;
     switch (BUILTIN_TYPE(obj)) {
       case T_STRING:
       case T_REGEXP: