diff --git a/composer.json b/composer.json
index d632c8991..fd4ddd963 100644
--- a/composer.json
+++ b/composer.json
@@ -24,8 +24,11 @@
         "doctrine/annotations": "~1.4",
         "symfony/console": "~3.0|~4.0"
     },
+    "conflict": {
+        "symfony/yaml": "<3.4"
+    },
     "require-dev": {
-        "symfony/yaml": "~3.0|~4.0",
+        "symfony/yaml": "~3.4|~4.0",
         "phpunit/phpunit": "^6.0"
     },
     "suggest": {