diff --git a/core/errors.rbs b/core/errors.rbs
index 4ffb373b8..4abbac120 100644
--- a/core/errors.rbs
+++ b/core/errors.rbs
@@ -624,7 +624,8 @@ class SystemCallError < StandardError
   # A subclass of SystemCallError can also be instantiated via the `new` method of
   # the subclass.  See Errno.
   #
-  def initialize: (string msg, Integer errno) -> void
+  def initialize: (Integer errno) -> void
+                | (string? msg, ?Integer? errno, ?_ToS func) -> void
 
   # <!--
   #   rdoc-file=error.c
@@ -641,7 +642,7 @@ class SystemCallError < StandardError
   # -->
   # Return this SystemCallError's error number.
   #
-  def errno: () -> Integer
+  def errno: () -> Integer?
 end
 
 # <!-- rdoc-file=error.c -->
diff --git a/test/stdlib/SystemCallError_test.rb b/test/stdlib/SystemCallError_test.rb
index 2b0b6a422..ed6d6cac0 100644
--- a/test/stdlib/SystemCallError_test.rb
+++ b/test/stdlib/SystemCallError_test.rb
@@ -1,26 +1,35 @@
 require_relative "test_helper"
 
-class SystemCallErrorTest < StdlibTest
-  target SystemCallError
-
-  def test_initialize
-    SystemCallError.new('hi', 0)
-    a = SystemCallError.new(ToStr.new('hi'), 0)
-    a.errno
-    a.message
+class SystemCallErrorSingletonTest < Test::Unit::TestCase
+  include TestHelper
+
+  testing 'singleton(::SystemCallError)'
+
+  def test_new
+    assert_send_type '(Integer) -> SystemCallError',
+                     SystemCallError, :new, Errno::ENOENT::Errno
+
+    with_string do |message|
+      assert_send_type '(string) -> SystemCallError',
+                       SystemCallError, :new, message
+      assert_send_type '(string, Integer) -> SystemCallError',
+                       SystemCallError, :new, message, 0
+      assert_send_type '(string, Integer, _ToS) -> SystemCallError',
+                       SystemCallError, :new, message, Errno::ENOENT::Errno, ToS.new('path')
+    end
   end
+end
+
+class SystemCallErrorInstanceTest < Test::Unit::TestCase
+  include TestHelper
+
+  testing '::SystemCallError'
 
   def test_errno
-    begin
-      raise Errno::ENOENT, 'test'
-    rescue SystemCallError => exception
-      exception.errno
-    end
+    error = SystemCallError.new(Errno::ENOENT::Errno)
+    assert_send_type '() -> Integer', error, :errno
 
-    begin
-      raise SystemCallError.new('test', 3)
-    rescue SystemCallError => exception
-      exception.errno
-    end
+    error = SystemCallError.new('test')
+    assert_send_type '() -> nil', error, :errno
   end
 end