diff --git a/nixos/modules/services/x11/xserver.nix b/nixos/modules/services/x11/xserver.nix
index 09fcdd0b72b..01e1659b30b 100644
--- a/nixos/modules/services/x11/xserver.nix
+++ b/nixos/modules/services/x11/xserver.nix
@@ -648,34 +648,25 @@ in
 
     services.xserver.xkbDir = mkDefault "${pkgs.xkeyboard_config}/etc/X11/xkb";
 
-    system.extraDependencies = [
-      (pkgs.runCommand "xkb-layouts-exist" {
-            layouts=cfg.layout;
-        } ''
-        missing=()
-        while read -d , layout
-        do
-          [[ -f "${cfg.xkbDir}/symbols/$layout" ]] || missing+=($layout)
-        done <<< "$layouts,"
-        if [[ ''${#missing[@]} -eq 0 ]]
-        then
-          touch $out
-          exit 0
-        fi
+    system.extraDependencies = singleton (pkgs.runCommand "xkb-layouts-exist" {
+      inherit (cfg) layout xkbDir;
+    } ''
+      sed -n -e ':i /^! \(layout\|variant\) *$/ {
+        :l; n; /^!/bi; s/^ *\([^ ]\+\).*/\1/p; tl
+      }' "$xkbDir/rules/base.lst" | grep -qxF "$layout" && exec touch "$out"
 
-        cat >&2 <<EOF
+      cat >&2 <<-EOF
 
-        Some of the selected keyboard layouts do not exist:
+      The selected keyboard layout definition does not exist:
 
-          ''${missing[@]}
+        $layout
 
-        Set services.xserver.layout to the name of an existing keyboard
-        layout (check ${cfg.xkbDir}/symbols for options).
+      Set \`services.xserver.layout' to the name of an existing keyboard
+      layout (check $xkbDir/rules/base.lst for options).
 
-        EOF
-        exit -1
-      '')
-    ];
+      EOF
+      exit 1
+    '');
 
     services.xserver.config =
       ''