diff --git a/src/PhpFileFinder.php b/src/PhpFileFinder.php
index <HASH>..<HASH> 100644
--- a/src/PhpFileFinder.php
+++ b/src/PhpFileFinder.php
@@ -22,7 +22,7 @@ class PhpFileFinder
             ), '/^.+\.php$/i', \RecursiveRegexIterator::GET_MATCH
         );
         foreach ($regexIterator as $fileName) {
-            $collection->add(new PhpFile(new \SplFileInfo($fileName[0])));
+            $collection = $collection->add(new PhpFile(new \SplFileInfo($fileName[0])));
         }
 
         return $collection;
Fix bug where immutable collection was treated like mutable