diff --git a/acme/autocert/autocert.go b/acme/autocert/autocert.go
index 4c2fc07..a50d9bf 100644
--- a/acme/autocert/autocert.go
+++ b/acme/autocert/autocert.go
@@ -69,7 +69,7 @@ func HostWhitelist(hosts ...string) HostPolicy {
 	}
 	return func(_ context.Context, host string) error {
 		if !whitelist[host] {
-			return errors.New("acme/autocert: host not configured")
+			return fmt.Errorf("acme/autocert: host %q not configured in HostWhitelist", host)
 		}
 		return nil
 	}