diff --git a/composer.lock b/composer.lock
index 29a466328d..0810d2872c 100644
--- a/composer.lock
+++ b/composer.lock
@@ -1424,32 +1424,32 @@
         },
         {
             "name": "symfony/debug",
-            "version": "v4.0.0",
+            "version": "v3.4.0",
             "source": {
                 "type": "git",
                 "url": "https://github.com/symfony/debug.git",
-                "reference": "26a15dab86c3820473716be4f846eac774ad4ad9"
+                "reference": "fb2001e5d85f95d8b6ab94ae3be5d2672df128fd"
             },
             "dist": {
                 "type": "zip",
-                "url": "https://api.github.com/repos/symfony/debug/zipball/26a15dab86c3820473716be4f846eac774ad4ad9",
-                "reference": "26a15dab86c3820473716be4f846eac774ad4ad9",
+                "url": "https://api.github.com/repos/symfony/debug/zipball/fb2001e5d85f95d8b6ab94ae3be5d2672df128fd",
+                "reference": "fb2001e5d85f95d8b6ab94ae3be5d2672df128fd",
                 "shasum": ""
             },
             "require": {
-                "php": "^7.1.3",
+                "php": "^5.5.9|>=7.0.8",
                 "psr/log": "~1.0"
             },
             "conflict": {
-                "symfony/http-kernel": "<3.4"
+                "symfony/http-kernel": ">=2.3,<2.3.24|~2.4.0|>=2.5,<2.5.9|>=2.6,<2.6.2"
             },
             "require-dev": {
-                "symfony/http-kernel": "~3.4|~4.0"
+                "symfony/http-kernel": "~2.8|~3.0|~4.0"
             },
             "type": "library",
             "extra": {
                 "branch-alias": {
-                    "dev-master": "4.0-dev"
+                    "dev-master": "3.4-dev"
                 }
             },
             "autoload": {
@@ -1476,7 +1476,7 @@
             ],
             "description": "Symfony Debug Component",
             "homepage": "https://symfony.com",
-            "time": "2017-11-21T09:27:49+00:00"
+            "time": "2017-11-21T09:01:46+00:00"
         },
         {
             "name": "symfony/dependency-injection",
diff --git a/core/composer.json b/core/composer.json
index 1226c622c2..1c370cd05f 100644
--- a/core/composer.json
+++ b/core/composer.json
@@ -7,6 +7,7 @@
         "php": "^5.5.9|>=7.0.8",
         "symfony/class-loader": "~3.4.0",
         "symfony/console": "~3.4.0",
+        "symfony/debug": "~3.4.0",
         "symfony/dependency-injection": "~3.4.0",
         "symfony/event-dispatcher": "~3.4.0",
         "symfony/http-foundation": "~3.4.0",