diff --git a/stapler/cert_manager.py b/stapler/cert_manager.py
index 804cf6d..f553df6 100644
--- a/stapler/cert_manager.py
+++ b/stapler/cert_manager.py
@@ -192,7 +192,7 @@ class CertManager:
         _: ssl.SSLSocket,
         /,
     ) -> None | int:
-        if host is None:
+        if host is None or not valid_host(host):
             return None
         self.logger.debug("servername callback: %s", host)
         if not self.exists(host) and not self.create_or_update(host):