diff --git a/ChangeLog b/ChangeLog
index 2967c6c88c..dc9a359213 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -3,6 +3,8 @@ Thu Jan 10 10:15:03 2008  Yukihiro Matsumoto  <matz@ruby-lang.org>
 	* io.c (io_encoding_set): IO.pipe("euc-jp", nil) should work as
 	  IO.pipe("euc-jp", nil).  [ruby-dev:33000]
 
+	* io.c (io_encoding_set): handle nil for v1.
+
 Thu Jan 10 02:41:22 2008  Yukihiro Matsumoto  <matz@ruby-lang.org>
 
 	* io.c (rb_io_binmode): should not alter encoding.  [ruby-dev:32918]
diff --git a/io.c b/io.c
index 2fc199d861..4b499bc6e6 100644
--- a/io.c
+++ b/io.c
@@ -5627,12 +5627,17 @@ io_encoding_set(rb_io_t *fptr, int argc, VALUE v1, VALUE v2)
 	fptr->enc = rb_to_encoding(v2);
     }
     else if (argc == 1) {
-	VALUE tmp = rb_check_string_type(v1);
-	if (!NIL_P(tmp)) {
-	    mode_enc(fptr, StringValueCStr(tmp));
+	if if (NIL_P(v1)) {
+	    fptr->enc = 0;
 	}
 	else {
-	    fptr->enc = rb_to_encoding(v1);
+	    VALUE tmp = rb_check_string_type(v1);
+	    if (!NIL_P(tmp)) {
+		mode_enc(fptr, StringValueCStr(tmp));
+	    }
+	    else {
+		fptr->enc = rb_to_encoding(v1);
+	    }
 	}
     }
 }