diff --git a/ChangeLog b/ChangeLog
index d0e5df7d1e..2e90d592a5 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Wed Feb 22 17:36:22 2012  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* io.c (rb_io_s_foreach): argument check before making Enumerator.
+	  [ruby-dev:31525]
+
 Wed Feb 22 17:07:35 2012  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* io.c (rb_io_s_foreach): return enumerator including kerword
diff --git a/io.c b/io.c
index add37237ab..9301dc6814 100644
--- a/io.c
+++ b/io.c
@@ -8969,10 +8969,11 @@ static VALUE
 rb_io_s_foreach(int argc, VALUE *argv, VALUE self)
 {
     VALUE opt;
+    int orig_argc = argc;
     struct foreach_arg arg;
 
-    RETURN_ENUMERATOR(self, argc, argv);
     argc = rb_scan_args(argc, argv, "13:", NULL, NULL, NULL, NULL, &opt);
+    RETURN_ENUMERATOR(self, orig_argc, argv);
     open_key_args(argc, argv, opt, &arg);
     if (NIL_P(arg.io)) return Qnil;
     return rb_ensure(io_s_foreach, (VALUE)&arg, rb_io_close, arg.io);
diff --git a/test/ruby/test_io.rb b/test/ruby/test_io.rb
index 54253c1f99..a08c003403 100644
--- a/test/ruby/test_io.rb
+++ b/test/ruby/test_io.rb
@@ -1605,6 +1605,13 @@ End
     IO.foreach(t.path, "b", 3) {|x| a << x }
     assert_equal(["foo", "\nb", "ar\n", "b", "az\n"], a)
 
+    bug = '[ruby-dev:31525]'
+    assert_raise(ArgumentError, bug) {IO.foreach}
+
+    a = nil
+    assert_nothing_raised(ArgumentError, bug) {a = IO.foreach(t.path).to_a}
+    assert_equal(["foo\n", "bar\n", "baz\n"], a, bug)
+
     bug6054 = '[ruby-dev:45267]'
     e = assert_raise(IOError, bug6054) {IO.foreach(t.path, mode:"w").next}
     assert_match(/not opened for reading/, e.message, bug6054)