diff --git a/ChangeLog b/ChangeLog
index 9df43f2f05..4817f97713 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Fri Nov 19 10:32:36 2004  Shugo Maeda  <shugo@ruby-lang.org>
+
+	* ext/readline/readline.c (readline_s_set_completion_append_character):
+	  accept nil.  [ruby-core:03765]
+
 Fri Nov 19 00:59:31 2004  Yukihiro Matsumoto  <matz@ruby-lang.org>
 
 	* string.c (str_gsub): internal buffer should not be listed by
diff --git a/ext/readline/readline.c b/ext/readline/readline.c
index 8812524a67..e3a1dcbf02 100644
--- a/ext/readline/readline.c
+++ b/ext/readline/readline.c
@@ -204,11 +204,16 @@ readline_s_set_completion_append_character(self, str)
 {
 #ifdef READLINE_21_OR_LATER
     rb_secure(4);
-    SafeStringValue(str);
-    if (NIL_P(str) || RSTRING(str)->len == 0) {
+    if (NIL_P(str)) {
 	rl_completion_append_character = '\0';
-    } else {
-	rl_completion_append_character = RSTRING(str)->ptr[0];
+    }
+    else {
+	SafeStringValue(str);
+	if (RSTRING(str)->len == 0) {
+	    rl_completion_append_character = '\0';
+	} else {
+	    rl_completion_append_character = RSTRING(str)->ptr[0];
+	}
     }
 
     return self;
diff --git a/test/readline/test_readline.rb b/test/readline/test_readline.rb
index f41f3584cb..80c007c063 100644
--- a/test/readline/test_readline.rb
+++ b/test/readline/test_readline.rb
@@ -38,6 +38,15 @@ class TestReadline < Test::Unit::TestCase
     end
   end
 
+  def test_completion_append_character
+    Readline.completion_append_character = nil
+    assert_equal(nil, Readline.completion_append_character)
+    Readline.completion_append_character = "x"
+    assert_equal("x", Readline.completion_append_character)
+    Readline.completion_append_character = "xyz"
+    assert_equal("x", Readline.completion_append_character)
+  end
+
   private
 
   def replace_stdio(stdin, stdout)