diff --git a/mrbgems/mruby-io/src/io.c b/mrbgems/mruby-io/src/io.c
index e5a2fcc04..5dd89ce68 100644
--- a/mrbgems/mruby-io/src/io.c
+++ b/mrbgems/mruby-io/src/io.c
@@ -706,6 +706,9 @@ io_init(mrb_state *mrb, mrb_value io)
 
   mode = opt = mrb_nil_value();
 
+  if (mrb_block_given_p(mrb)) {
+    mrb_warn(mrb, "File.new() does not take block; use File.open() instead");
+  }
   mrb_get_args(mrb, "i|oH", &fd, &mode, &opt);
   switch (fd) {
     case 0: /* STDIN_FILENO */
diff --git a/mrbgems/mruby-io/test/io.rb b/mrbgems/mruby-io/test/io.rb
index 024a71acd..d146d6d8b 100644
--- a/mrbgems/mruby-io/test/io.rb
+++ b/mrbgems/mruby-io/test/io.rb
@@ -16,14 +16,15 @@ def assert_io_open(meth)
       io1.close
     end
 
-    io2 = IO.__send__(meth, IO.sysopen($mrbtest_io_rfname))do |io|
-      if meth == :open
-        assert_equal $mrbtest_io_msg, io.read
-      else
-        flunk "IO.#{meth} does not take block"
+    if meth == :open
+      io2 = IO.__send__(meth, IO.sysopen($mrbtest_io_rfname))do |io|
+        if meth == :open
+          assert_equal $mrbtest_io_msg, io.read
+        else
+          flunk "IO.#{meth} does not take block"
+        end
       end
     end
-    io2.close unless meth == :open
 
     assert_raise(RuntimeError) { IO.__send__(meth, 1023) } # For Windows
     assert_raise(RuntimeError) { IO.__send__(meth, 1 << 26) }