diff --git a/README.md b/README.md
index 6edfae2..082a14e 100644
--- a/README.md
+++ b/README.md
@@ -5,7 +5,7 @@ Checks the expiration dates of domains.
 
 ## Installation and usage
 
-PHP 7.3 and [composer](https://getcomposer.org/) required.
+PHP 7.3, [composer](https://getcomposer.org/) and `whois` required.
 
 ```
 $ git clone https://gitnet.fr/deblan/domain-expiration.git