diff --git a/system/Autoloader/FileLocatorCached.php b/system/Autoloader/FileLocatorCached.php
index 0aa267b2a84b..28b7302f1dce 100644
--- a/system/Autoloader/FileLocatorCached.php
+++ b/system/Autoloader/FileLocatorCached.php
@@ -163,14 +163,16 @@ public function listNamespaceFiles(string $prefix, string $path): array
 
     public function locateFile(string $file, ?string $folder = null, string $ext = 'php'): false|string
     {
-        if (isset($this->cache['locateFile'][$file][$folder][$ext])) {
-            return $this->cache['locateFile'][$file][$folder][$ext];
+        $folderKey = $folder ?? '';
+
+        if (isset($this->cache['locateFile'][$file][$folderKey][$ext])) {
+            return $this->cache['locateFile'][$file][$folderKey][$ext];
         }
 
-        $files = $this->locator->locateFile($file, $folder, $ext);
+        $files = $this->locator->locateFile($file, $folderKey, $ext);
 
-        $this->cache['locateFile'][$file][$folder][$ext] = $files;
-        $this->cacheUpdated                              = true;
+        $this->cache['locateFile'][$file][$folderKey][$ext] = $files;
+        $this->cacheUpdated                                 = true;
 
         return $files;
     }
diff --git a/tests/system/Validation/RulesTest.php b/tests/system/Validation/RulesTest.php
index 8bb6cab3ce77..14c99d5d9d78 100644
--- a/tests/system/Validation/RulesTest.php
+++ b/tests/system/Validation/RulesTest.php
@@ -779,7 +779,7 @@ public static function provideRequiredWithAndOtherRuleWithValueZero(): iterable
     }
 
     #[DataProvider('provideRequiredWithout')]
-    public function testRequiredWithout(?string $field, ?string $check, bool $expected): void
+    public function testRequiredWithout(string $field, ?string $check, bool $expected): void
     {
         $data = [
             'foo'   => 'bar',
@@ -802,8 +802,8 @@ public static function provideRequiredWithout(): iterable
         yield from [
             ['nope', 'bars', false],
             ['foo', 'nope', true],
-            [null, null, false],
-            [null, 'foo', true],
+            ['', null, false],
+            ['', 'foo', true],
             ['foo', null, true],
             [
                 'array.emptyField1',