diff --git a/docs/contributing.rst b/docs/contributing.rst
index 229c63841..d95cbf4c0 100644
--- a/docs/contributing.rst
+++ b/docs/contributing.rst
@@ -186,7 +186,7 @@ Authenticators
 --------------
 
 Authenticators are plugins that prove control of a domain name by solving a
-challenge provided by the ACME server. ACME currently defines three types of
+challenge provided by the ACME server. ACME currently defines four types of
 challenges: HTTP, TLS-SNI (deprecated), TLS-ALPR, and DNS, represented by classes in `acme.challenges`.
 An authenticator plugin should implement support for at least one challenge type.