diff --git a/system/Test/CIUnitTestCase.php b/system/Test/CIUnitTestCase.php
index d86e543a9a61..ced4f13c6c78 100644
--- a/system/Test/CIUnitTestCase.php
+++ b/system/Test/CIUnitTestCase.php
@@ -69,7 +69,9 @@ abstract class CIUnitTestCase extends TestCase
      *
      * @var list<string> array of methods
      */
-    protected $tearDownMethods = [];
+    protected $tearDownMethods = [
+        'resetIsWindowsMock',
+    ];
 
     /**
      * Store of identified traits.
@@ -318,6 +320,16 @@ protected function resetServices(bool $initAutoloader = true)
         Services::reset($initAutoloader);
     }
 
+    /**
+     * Resets the mocked is_windows() function back to default state.
+     */
+    protected function resetIsWindowsMock(): void
+    {
+        if (function_exists('is_windows')) {
+            is_windows(null);
+        }
+    }
+
     /**
      * Injects the mock Cache driver to prevent filesystem collisions.
      *
diff --git a/tests/system/CommonFunctionsTest.php b/tests/system/CommonFunctionsTest.php
index b4901a5e8d93..f1ce25c04aa1 100644
--- a/tests/system/CommonFunctionsTest.php
+++ b/tests/system/CommonFunctionsTest.php
@@ -863,7 +863,7 @@ public function testIsWindowsUsingMock(): void
         $this->assertFalse(is_windows());
         $this->assertNotTrue(is_windows());
 
-        is_windows();
+        is_windows(null);
         $this->assertSame(str_contains(php_uname(), 'Windows'), is_windows());
         $this->assertSame(defined('PHP_WINDOWS_VERSION_MAJOR'), is_windows());
     }