diff --git a/src/SourceFile.php b/src/SourceFile.php
index 8f37cbe..33bc550 100644
--- a/src/SourceFile.php
+++ b/src/SourceFile.php
@@ -32,7 +32,7 @@ public static function fromString(string $content, ?UriInterface $sourceUrl = nu
         return new SourceFile($content, $sourceUrl);
     }
 
-    public function __construct(string $content, ?UriInterface $sourceUrl = null)
+    private function __construct(string $content, ?UriInterface $sourceUrl = null)
     {
         $this->string = $content;
         $this->sourceUrl = $sourceUrl;