diff --git a/ChangeLog b/ChangeLog
index 3725f09b57..197a4f8020 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,12 @@
+Fri Dec 12 19:29:07 2008  Yuki Sonoda (Yugui)  <yugui@yugui.jp>
+
+	* encoding.c (rb_enc_set_default_external): default_internal can be
+	  nil, but default_external cannot.
+
+	* encoding.c (rb_set_default_internal): adds rdoc.
+
+	* encoding.c (enc_find): ditto.
+
 Fri Dec 12 19:24:47 2008  Yuki Sonoda (Yugui)  <yugui@yugui.jp>
 
 	* common.mk (run, runruby, parse, gdb, gdb-ruby): you can also run
diff --git a/encoding.c b/encoding.c
index 910768d56a..f802850f3b 100644
--- a/encoding.c
+++ b/encoding.c
@@ -905,6 +905,9 @@ enc_list(VALUE klass)
  *   Encoding.find("US-ASCII")  => #<Encoding:US-ASCII>
  *   Encoding.find(:Shift_JIS)  => #<Encoding:Shift_JIS>
  *
+ * An ArgumentError is raised when no encoding with <i>name</i>.
+ * Only +Encoding.find("internal")+ however returns nil when no encoding named "internal",
+ * in other words, when Ruby has no default internal encoding.
  */
 static VALUE
 enc_find(VALUE klass, VALUE enc)
@@ -1102,6 +1105,9 @@ get_default_external(VALUE klass)
 void
 rb_enc_set_default_external(VALUE encoding)
 {
+    if (NIL_P(encoding)) {
+        rb_raise(rb_eArgError, "default external can not be nil");
+    }
     enc_set_default_encoding(&default_external, encoding,
 			     "external", ENCINDEX_US_ASCII);
 }
@@ -1161,9 +1167,10 @@ rb_enc_set_default_internal(VALUE encoding)
 
 /*
  * call-seq:
- *   Encoding.default_internal = enc
+ *   Encoding.default_internal = enc or nil
  *
  * Sets default internal encoding.
+ * Or removes default internal encoding when passed nil.
  */
 static VALUE
 set_default_internal(VALUE klass, VALUE encoding)