diff --git a/src/InMemoryFileFetcher.php b/src/InMemoryFileFetcher.php
index <HASH>..<HASH> 100644
--- a/src/InMemoryFileFetcher.php
+++ b/src/InMemoryFileFetcher.php
@@ -14,6 +14,8 @@ use InvalidArgumentException;
  */
 class InMemoryFileFetcher implements FileFetcher {
 
+	private $files;
+
 	/**
 	 * @param string[] $files
 	 * @throws InvalidArgumentException
Explicitly define field in InMemoryFileFetcher