diff --git a/composer.json b/composer.json
index fb6b2899..dce2c54b 100644
--- a/composer.json
+++ b/composer.json
@@ -11,6 +11,7 @@
 		"php": ">=7.0"
 	},
 	"require-dev": {
+		"ext-mbstring": "*",
 		"dealerdirect/phpcodesniffer-composer-installer": "^0.7.0",
 		"squizlabs/php_codesniffer": "^3.7",
 		"wp-coding-standards/wpcs": "^3.1",