diff --git a/nixos/modules/services/networking/wireguard.nix b/nixos/modules/services/networking/wireguard.nix
index b08f1015e8b8..8b025228cc1f 100644
--- a/nixos/modules/services/networking/wireguard.nix
+++ b/nixos/modules/services/networking/wireguard.nix
@@ -461,7 +461,7 @@ let
 
           ${ipPreMove} link add dev "${name}" type wireguard
           ${optionalString (values.interfaceNamespace != null && values.interfaceNamespace != values.socketNamespace) ''${ipPreMove} link set "${name}" netns "${ns}"''}
-          ${optionalString (values.mtu != null) ''${ipPreMove} link set "${name}" mtu ${toString values.mtu}''}
+          ${optionalString (values.mtu != null) ''${ipPostMove} link set "${name}" mtu ${toString values.mtu}''}
 
           ${concatMapStringsSep "\n" (ip:
             ''${ipPostMove} address add "${ip}" dev "${name}"''
diff --git a/nixos/tests/wireguard/namespaces.nix b/nixos/tests/wireguard/namespaces.nix
index 1790c45bb1f6..d0eb009e1107 100644
--- a/nixos/tests/wireguard/namespaces.nix
+++ b/nixos/tests/wireguard/namespaces.nix
@@ -39,6 +39,7 @@ import ../make-test-python.nix ({ pkgs, lib, kernelPackages ? null, ... } : {
         preSetup = ''
           ip netns add ${interfaceNamespace}
         '';
+        mtu = 1280;
         inherit interfaceNamespace;
       };
     };