diff --git a/lib/options.nix b/lib/options.nix
index a16a980398d6..d2eae3ae3a98 100644
--- a/lib/options.nix
+++ b/lib/options.nix
@@ -101,7 +101,7 @@ rec {
   mergeOneOption = loc: defs:
     if defs == [] then abort "This case should never happen."
     else if length defs != 1 then
-      throw "The unique option `${showOption loc}' is defined multiple times, in ${showFiles (getFiles defs)}."
+      throw "The unique option `${showOption loc}' is defined multiple times, in:\n - ${concatStringsSep "\n - " (getFiles defs)}."
     else (head defs).value;
 
   /* "Merge" option definitions by checking that they all have the same value. */
diff --git a/nixos/modules/virtualisation/containers.nix b/nixos/modules/virtualisation/containers.nix
index 97243e3304ee..2235eec9d95a 100644
--- a/nixos/modules/virtualisation/containers.nix
+++ b/nixos/modules/virtualisation/containers.nix
@@ -465,20 +465,24 @@ in
                 merge = loc: defs: (import ../../lib/eval-config.nix {
                   inherit system;
                   modules =
-                    let extraConfig =
-                      { boot.isContainer = true;
-                        networking.hostName = mkDefault name;
-                        networking.useDHCP = false;
-                        assertions = [
-                          {
-                            assertion =  config.privateNetwork -> stringLength name < 12;
-                            message = ''
-                              Container name `${name}` is too long: When `privateNetwork` is enabled, container names can
-                              not be longer than 11 characters, because the container's interface name is derived from it.
-                              This might be fixed in the future. See https://github.com/NixOS/nixpkgs/issues/38509
-                            '';
-                          }
-                        ];
+                    let
+                      extraConfig = {
+                        _file = "module at ${__curPos.file}:${toString __curPos.line}";
+                        config = {
+                          boot.isContainer = true;
+                          networking.hostName = mkDefault name;
+                          networking.useDHCP = false;
+                          assertions = [
+                            {
+                              assertion =  config.privateNetwork -> stringLength name < 12;
+                              message = ''
+                                Container name `${name}` is too long: When `privateNetwork` is enabled, container names can
+                                not be longer than 11 characters, because the container's interface name is derived from it.
+                                This might be fixed in the future. See https://github.com/NixOS/nixpkgs/issues/38509
+                              '';
+                            }
+                          ];
+                        };
                       };
                     in [ extraConfig ] ++ (map (x: x.value) defs);
                   prefix = [ "containers" name ];