diff --git a/phpBB/phpbb/filesystem/filesystem.php b/phpBB/phpbb/filesystem/filesystem.php
index 65585738b2..2931286811 100644
--- a/phpBB/phpbb/filesystem/filesystem.php
+++ b/phpBB/phpbb/filesystem/filesystem.php
@@ -600,7 +600,7 @@ class filesystem implements filesystem_interface
 	 *
 	 * @deprecated 3.3.0-a1 (To be removed: 4.0.0)
 	 *
-	 * @param string	$path
+	 * @param ?string	$path
 	 * @return bool|string
 	 */
 	protected function phpbb_own_realpath($path)
diff --git a/phpBB/phpbb/filesystem/filesystem_interface.php b/phpBB/phpbb/filesystem/filesystem_interface.php
index 6d0d35d72f..fa7950dec3 100644
--- a/phpBB/phpbb/filesystem/filesystem_interface.php
+++ b/phpBB/phpbb/filesystem/filesystem_interface.php
@@ -239,7 +239,7 @@ interface filesystem_interface
 	 * Try to resolve realpath when PHP's realpath is not available, or
 	 * known to be buggy.
 	 *
-	 * @param string	$path	Path to resolve
+	 * @param ?string	$path	Path to resolve
 	 *
 	 * @return string	Resolved path
 	 */
diff --git a/phpBB/phpbb/storage/adapter/local.php b/phpBB/phpbb/storage/adapter/local.php
index 94990a6bf7..ebd8b12ffe 100644
--- a/phpBB/phpbb/storage/adapter/local.php
+++ b/phpBB/phpbb/storage/adapter/local.php
@@ -230,8 +230,9 @@ class local implements adapter_interface, stream_interface
 	 */
 	protected function ensure_directory_exists($path)
 	{
-		$path = dirname($this->root_path . $this->get_path($path) . $this->get_filename($path));
-		$path = filesystem_helper::make_path_relative($path, $this->root_path);
+		$absolute_root_path = filesystem_helper::realpath($this->root_path) . DIRECTORY_SEPARATOR;
+		$path = dirname($absolute_root_path . $this->get_path($path) . $this->get_filename($path));
+		$path = filesystem_helper::make_path_relative($path, $absolute_root_path);
 
 		if (!$this->exists($path))
 		{
@@ -254,11 +255,11 @@ class local implements adapter_interface, stream_interface
 
 			do
 			{
-				$parts = explode('/', $path);
+				$parts = explode(DIRECTORY_SEPARATOR, $path);
 				$parts = array_slice($parts, 0, -1);
-				$path = implode('/', $parts);
+				$path = implode(DIRECTORY_SEPARATOR, $parts);
 			}
-			while ($path && @rmdir($dirpath . '/' . $path));
+			while ($path && @rmdir($dirpath . DIRECTORY_SEPARATOR . $path));
 		}
 	}
 
diff --git a/tests/filesystem/realpath_test.php b/tests/filesystem/realpath_test.php
index 145ded7cfa..2b14758ffe 100644
--- a/tests/filesystem/realpath_test.php
+++ b/tests/filesystem/realpath_test.php
@@ -42,6 +42,7 @@ class phpbb_filesystem_realpath_test extends phpbb_test_case
 			array(__DIR__ . '/../filesystem/../filesystem', __DIR__),
 			array(__DIR__ . '/././', __DIR__),
 			array(__DIR__ . '/non_existent', false),
+			array(null, getcwd()),
 
 			array(__FILE__, __FILE__),
 			array(__FILE__ . '../', false),