@@ -851,9 +851,11 @@ io_s_sysopen(mrb_state *mrb, mrb_value klass)
 {
   mrb_value path = mrb_nil_value();
   mrb_value mode = mrb_nil_value();
-  mrb_int perm = -1;
+  mrb_value perm_v = mrb_nil_value();
 
-  mrb_get_args(mrb, "S|oi", &path, &mode, &perm);
+  /* a nil permission is the default one, as an omitted one is */
+  mrb_get_args(mrb, "S|oo", &path, &mode, &perm_v);
+  mrb_int perm = mrb_nil_p(perm_v) ? -1 : mrb_as_int(mrb, perm_v);
   if (perm < 0) {
     perm = 0666;
   }
 
@@ -475,6 +475,17 @@ def to_s
   end
 end
 
+assert('IO.sysopen with a nil permission') do
+  fd = IO.sysopen $mrbtest_io_rfname, "r", nil
+  io = IO.new(fd)
+  assert_equal $mrbtest_io_msg, io.read
+  io.close
+
+  File.open($mrbtest_io_wfname, "w", nil) { |f| f.write "abc" }
+  assert_equal "abc", File.read($mrbtest_io_wfname)
+  assert_raise(TypeError) { IO.sysopen $mrbtest_io_rfname, "r", "0644" }
+end
+
 assert('IO.sysopen, IO#sysread') do
   fd = IO.sysopen $mrbtest_io_rfname
   io = IO.new(fd)
Original file line number	Diff line number	Diff line change
`@@ -851,9 +851,11 @@ io_s_sysopen(mrb_state *mrb, mrb_value klass)`
`851`	`851`	`{`
`852`	`852`	`mrb_value path = mrb_nil_value();`
`853`	`853`	`mrb_value mode = mrb_nil_value();`
`854`		`- mrb_int perm = -1;`
	`854`	`+ mrb_value perm_v = mrb_nil_value();`
`855`	`855`
`856`		`- mrb_get_args(mrb, "S\|oi", &path, &mode, &perm);`
	`856`	`+ /* a nil permission is the default one, as an omitted one is */`
	`857`	`+ mrb_get_args(mrb, "S\|oo", &path, &mode, &perm_v);`
	`858`	`+ mrb_int perm = mrb_nil_p(perm_v) ? -1 : mrb_as_int(mrb, perm_v);`
`857`	`859`	`if (perm < 0) {`
`858`	`860`	`perm = 0666;`
`859`	`861`	`}`