diff --git a/phpBB/composer.json b/phpBB/composer.json
index 31ad38a6b4..d5acd3112c 100644
--- a/phpBB/composer.json
+++ b/phpBB/composer.json
@@ -27,6 +27,7 @@
 	"require": {
 		"php": "^7.1.3",
 		"ext-json": "*",
+		"ext-mbstring": "*",
 		"bantu/ini-get-wrapper": "~1.0",
 		"google/recaptcha": "~1.1",
 		"guzzlehttp/guzzle": "~6.3",