diff --git a/composer.json b/composer.json
index 6d33043..93a8887 100644
--- a/composer.json
+++ b/composer.json
@@ -13,7 +13,8 @@
     "ext-mbstring": "*"
   },
   "require-dev": {
-    "phpunit/phpunit": "^4.8"
+    "phpunit/phpunit": "^4.8",
+    "psr/http-message": "^1.0"
   },
   "config": {
     "bin-dir": "bin"