diff --git a/system/Config/BaseConfig.php b/system/Config/BaseConfig.php
index 0269194bdc4c..7daa6e1544c0 100644
--- a/system/Config/BaseConfig.php
+++ b/system/Config/BaseConfig.php
@@ -55,7 +55,7 @@ class BaseConfig
     /**
      * The modules configuration.
      *
-     * @var Modules
+     * @var Modules|null
      */
     protected static $moduleConfig;
 
@@ -74,15 +74,24 @@ public static function __set_state(array $array)
         return $obj;
     }
 
+    /**
+     * @internal For testing purposes only.
+     * @testTag
+     */
+    public static function setModules(Modules $modules): void
+    {
+        static::$moduleConfig = $modules;
+    }
+
     /**
      * Will attempt to get environment variables with names
      * that match the properties of the child class.
      *
      * The "shortPrefix" is the lowercase-only config class name.
      */
-    public function __construct(?Modules $modules = null)
+    public function __construct()
     {
-        static::$moduleConfig = $modules ?? new Modules();
+        static::$moduleConfig ??= new Modules();
 
         if (! static::$override) {
             return;
diff --git a/tests/system/Config/BaseConfigTest.php b/tests/system/Config/BaseConfigTest.php
index ca615602a276..76861ccb12ad 100644
--- a/tests/system/Config/BaseConfigTest.php
+++ b/tests/system/Config/BaseConfigTest.php
@@ -50,6 +50,7 @@ protected function setUp(): void
         }
 
         BaseConfig::$registrars = [];
+        BaseConfig::setModules(new Modules()); // reset to clean copy of Modules
     }
 
     public function testBasicValues(): void
@@ -276,7 +277,8 @@ public function testDiscoveryNotEnabledWillNotPopulateRegistrarsArray(): void
         $modules = $this->createMock(Modules::class);
         $modules->method('shouldDiscover')->with('registrars')->willReturn(false);
 
-        $config = new RegistrarConfig($modules);
+        RegistrarConfig::setModules($modules);
+        $config = new RegistrarConfig();
 
         $this->assertSame([], $config::$registrars);
     }