diff --git a/spec/win32_std_spec.cr b/spec/win32_std_spec.cr
index 7f48244fda1b..f3de76d829ea 100644
--- a/spec/win32_std_spec.cr
+++ b/spec/win32_std_spec.cr
@@ -152,7 +152,7 @@ require "./std/mime/multipart/builder_spec.cr"
 require "./std/mime/multipart/parser_spec.cr"
 require "./std/mime/multipart_spec.cr"
 require "./std/mime_spec.cr"
-# require "./std/mutex_spec.cr" (failed codegen)
+require "./std/mutex_spec.cr"
 require "./std/named_tuple_spec.cr"
 require "./std/number_spec.cr"
 require "./std/oauth/access_token_spec.cr"
diff --git a/src/crystal/system/win32/process.cr b/src/crystal/system/win32/process.cr
index 00167b230fdb..75c7e581e425 100644
--- a/src/crystal/system/win32/process.cr
+++ b/src/crystal/system/win32/process.cr
@@ -1,5 +1,6 @@
 require "c/processthreadsapi"
 require "c/handleapi"
+require "c/synchapi"
 require "process/shell"
 
 struct Crystal::System::Process
diff --git a/src/prelude.cr b/src/prelude.cr
index ea88f25215f1..b6f98b2dd25c 100644
--- a/src/prelude.cr
+++ b/src/prelude.cr
@@ -55,9 +55,7 @@ require "intrinsics"
 require "io"
 require "kernel"
 require "math/math"
-{% unless flag?(:win32) %}
-  require "mutex"
-{% end %}
+require "mutex"
 require "named_tuple"
 require "nil"
 require "humanize"
diff --git a/src/windows_stubs.cr b/src/windows_stubs.cr
index 667930897f75..17aba1c25289 100644
--- a/src/windows_stubs.cr
+++ b/src/windows_stubs.cr
@@ -1,31 +1,3 @@
-require "c/synchapi"
-
-class Mutex
-  enum Protection
-    Checked
-    Reentrant
-    Unchecked
-  end
-
-  def initialize(@protection : Protection = :checked)
-  end
-
-  def lock
-  end
-
-  def unlock
-  end
-
-  def synchronize
-    lock
-    begin
-      yield
-    ensure
-      unlock
-    end
-  end
-end
-
 enum Signal
   KILL = 0
 end