diff --git a/nixos/modules/services/x11/desktop-managers/kde5.nix b/nixos/modules/services/x11/desktop-managers/kde5.nix
index 237c1f5a189..125bce94aba 100644
--- a/nixos/modules/services/x11/desktop-managers/kde5.nix
+++ b/nixos/modules/services/x11/desktop-managers/kde5.nix
@@ -68,6 +68,9 @@ in
 
   config = mkIf (xcfg.enable && cfg.enable) {
 
+    warnings = optional config.services.xserver.desktopManager.kde4.enable
+      "KDE 4 should not be enabled at the same time as KDE 5";
+
     services.xserver.desktopManager.session = singleton {
       name = "kde5";
       bgSupport = true;