diff --git a/modules/services/x11/xserver.nix b/modules/services/x11/xserver.nix
index baf224975a2c..b4554ac5c525 100644
--- a/modules/services/x11/xserver.nix
+++ b/modules/services/x11/xserver.nix
@@ -342,11 +342,11 @@ in
 
         { assertion = if cfg.startOpenSSHAgent
                       then !cfg.startGnuPGAgent
-                      else if cfg.startGnuPGAgent
-                           then !cfg.startOpenSSHAgent
-                           else true;
+                      else (if cfg.startGnuPGAgent
+                            then !cfg.startOpenSSHAgent
+                            else true);
           message =
-            "The OpenSSH agent and GnuPG agent cannot be started both.  "
+            "The OpenSSH agent and GnuPG agent cannot be started both.  " +
             "Choose between `startOpenSSHAgent' and `startGnuPGAgent'.";
         }
       ];