diff --git a/ChangeLog b/ChangeLog
index cc3d30f726..f33053f933 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Sat Jun  7 23:47:35 2008  Akinori MUSHA  <knu@iDaemons.org>
+
+	* ext/zlib/zlib.c (rb_deflate_initialize, Init_zlib): Fix up
+	  initialize_copy; [ruby-list:45016].
+
 Sat Jun  7 22:15:02 2008  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* configure.in (VENDOR_DIR): use LIBDIR instead of PREFIX as well as
diff --git a/ext/zlib/zlib.c b/ext/zlib/zlib.c
index b971ea76db..b06ebe3fad 100644
--- a/ext/zlib/zlib.c
+++ b/ext/zlib/zlib.c
@@ -1116,10 +1116,12 @@ rb_deflate_initialize(int argc, VALUE *argv, VALUE obj)
 static VALUE
 rb_deflate_init_copy(VALUE self, VALUE orig)
 {
-    struct zstream *z1 = get_zstream(self);
-    struct zstream *z2 = get_zstream(orig);
+    struct zstream *z1, *z2;
     int err;
 
+    Data_Get_Struct(self, struct zstream, z1);
+    z2 = get_zstream(orig);
+
     err = deflateCopy(&z1->stream, &z2->stream);
     if (err != Z_OK) {
 	raise_zlib_error(err, 0);
@@ -3267,7 +3269,7 @@ void Init_zlib()
     rb_define_singleton_method(cDeflate, "deflate", rb_deflate_s_deflate, -1);
     rb_define_alloc_func(cDeflate, rb_deflate_s_allocate);
     rb_define_method(cDeflate, "initialize", rb_deflate_initialize, -1);
-    rb_define_method(cDeflate, "initialize_copy", rb_deflate_init_copy, 0);
+    rb_define_method(cDeflate, "initialize_copy", rb_deflate_init_copy, 1);
     rb_define_method(cDeflate, "deflate", rb_deflate_deflate, -1);
     rb_define_method(cDeflate, "<<", rb_deflate_addstr, 1);
     rb_define_method(cDeflate, "flush", rb_deflate_flush, -1);