diff --git a/composer.json b/composer.json
index 8baa766..ab95fa3 100644
--- a/composer.json
+++ b/composer.json
@@ -11,7 +11,7 @@
         }
     ],
     "require": {
-        "guzzlehttp/guzzle": "*",
+        "guzzlehttp/guzzle": "6.5.5",
         "php": ">=5.5"
     },
     "require-dev": {
@@ -28,4 +28,4 @@
             "sngrl\\PhpFirebaseCloudMessaging\\Tests\\": "tests/"
         }
     }
-}
\ No newline at end of file
+}