diff --git a/src/FileSystem/Finder/Iterator/RealPathFilterIterator.php b/src/FileSystem/Finder/Iterator/RealPathFilterIterator.php
index c1e40ab41..ecadd0e51 100644
--- a/src/FileSystem/Finder/Iterator/RealPathFilterIterator.php
+++ b/src/FileSystem/Finder/Iterator/RealPathFilterIterator.php
@@ -77,7 +77,7 @@ public function accept()
      *
      * @return string regexp corresponding to a given string or regexp
      */
-    protected function toRegex($str)
+    protected function toRegex(string $str): string
     {
         return $this->isRegex($str) ? $str : '/' . preg_quote($str, '/') . '/';
     }