diff --git a/composer.json b/composer.json
index c1897b8..3573c52 100644
--- a/composer.json
+++ b/composer.json
@@ -16,7 +16,7 @@
         "php"           : "^7.0",
         "sabre/dav"     : "~4.0.1",
         "twig/twig"     : "~1.8.0",
-        "symfony/yaml": "^4.3"
+        "symfony/yaml"  : "^3.4"
     },
     "require-dev" : {
         "sabre/cs"      : "~0.0.6"