diff --git a/composer.json b/composer.json
index 73c2922..c57f705 100644
--- a/composer.json
+++ b/composer.json
@@ -20,7 +20,8 @@
         }
     ],
     "require": {
-        "php" : "~7.0"
+        "php" : "~7.0",
+        "ext-dom": "*"
     },
     "require-dev": {
         "phpunit/phpunit" : ">=5.4.3",