diff --git a/nixos/modules/services/x11/desktop-managers/default.nix b/nixos/modules/services/x11/desktop-managers/default.nix
index f435e85f6b83..cce35aa28ba8 100644
--- a/nixos/modules/services/x11/desktop-managers/default.nix
+++ b/nixos/modules/services/x11/desktop-managers/default.nix
@@ -96,13 +96,13 @@ in
           else if any (w: w.name == defaultDM) cfg.session.list then
             defaultDM
           else
-            throw ''
-              Default desktop manager (${defaultDM}) not found.
-              Probably you want to change
-                services.xserver.desktopManager.default = "${defaultDM}";
-              to one of
+            builtins.trace ''
+              Default desktop manager (${defaultDM}) not found at evaluation time.
+              These are the known valid session names:
                 ${concatMapStringsSep "\n  " (w: "services.xserver.desktopManager.default = \"${w.name}\";") cfg.session.list}
-            '';
+              It's also possible the default can be found in one of these packages:
+                ${concatMapStringsSep "\n  " (p: p.name) config.services.xserver.displayManager.extraSessionFilePackages}
+            '' defaultDM;
       };
 
     };
diff --git a/nixos/modules/services/x11/display-managers/lightdm.nix b/nixos/modules/services/x11/display-managers/lightdm.nix
index 9b5f51fb4cf5..6be15d8cdf46 100644
--- a/nixos/modules/services/x11/display-managers/lightdm.nix
+++ b/nixos/modules/services/x11/display-managers/lightdm.nix
@@ -176,21 +176,13 @@ in
           LightDM auto-login requires services.xserver.displayManager.lightdm.autoLogin.user to be set
         '';
       }
-      { assertion = cfg.autoLogin.enable -> elem defaultSessionName dmcfg.session.names;
+      { assertion = cfg.autoLogin.enable -> dmDefault != "none" || wmDefault != "none";
         message = ''
           LightDM auto-login requires that services.xserver.desktopManager.default and
           services.xserver.windowMananger.default are set to valid values. The current
           default session: ${defaultSessionName} is not valid.
         '';
       }
-      { assertion = hasDefaultUserSession -> elem defaultSessionName dmcfg.session.names;
-        message = ''
-          services.xserver.desktopManager.default and
-          services.xserver.windowMananger.default are not set to valid
-          values. The current default session: ${defaultSessionName}
-          is not valid.
-        '';
-      }
       { assertion = !cfg.greeter.enable -> (cfg.autoLogin.enable && cfg.autoLogin.timeout == 0);
         message = ''
           LightDM can only run without greeter if automatic login is enabled and the timeout for it