diff --git a/composer.json b/composer.json
index a5e5db0..ceac334 100644
--- a/composer.json
+++ b/composer.json
@@ -14,7 +14,8 @@
         "php": "^7.1",
         "ext-intl": "*",
         "ext-mbstring": "*",
-        "ext-pcre": "*"
+        "ext-pcre": "*",
+        "lib-icu": ">=4.6"
     },
     "require-dev": {
         "friendsofphp/php-cs-fixer": "^2.4",