nixos/systemd: Custom error when mixing list/non-list defs

nixos/lib/systemd-unit-options.nix

@@ -20,10 +20,15 @@ in rec {
     merge = loc: defs:
       let
         defs' = filterOverrides defs;
-        defs'' = getValues defs';
       in
-        if isList (head defs'')
-        then concatLists defs''
+        if isList (head defs').value
+        then concatMap (def:
+          if builtins.typeOf def.value == "list"
+          then def.value
+          else
+            throw "The definitions for systemd unit options should be either all lists, representing repeatable options, or all non-lists, but for the option ${showOption loc}, the definitions are a mix of list and non-list ${lib.options.showDefs defs'}"
+        ) defs'
+
         else mergeEqualOption loc defs';
   };