diff --git a/oven/configuration.nix b/oven/configuration.nix
index 9e74f9c..ce18e43 100644
--- a/oven/configuration.nix
+++ b/oven/configuration.nix
@@ -58,10 +58,10 @@ in
     firewall = {
       allowedTCPPorts =
         map (port: port.num)
-          (filter (port: port.proto == "tcp") vars.ovenNat.forwardPorts);
+          (builtins.filter (port: port.proto == "tcp") vars.ovenNat.forwardPorts);
       allowedUDPPorts =
         map (port: port.num)
-          (filter (port: port.proto == "udp") vars.ovenNat.forwardPorts);
+          (builtins.filter (port: port.proto == "udp") vars.ovenNat.forwardPorts);
     };
   };
 
diff --git a/pastila/configuration.nix b/pastila/configuration.nix
index 16baab3..6920763 100644
--- a/pastila/configuration.nix
+++ b/pastila/configuration.nix
@@ -41,10 +41,10 @@ in
   # Open ports in the firewall.
   networking.firewall.allowedTCPPorts =
     map (port: port.num)
-      (filter (port: port.proto == "tcp") vars.ovenNat.forwardPorts);
+      (builtins.filter (port: port.proto == "tcp") vars.ovenNat.forwardPorts);
   networking.firewall.allowedUDPPorts =
     map (port: port.num)
-      (filter (port: port.proto == "udp") vars.ovenNat.forwardPorts);
+      (builtins.filter (port: port.proto == "udp") vars.ovenNat.forwardPorts);
 
   # Copy the NixOS configuration file and link it from the resulting system
   # (/run/current-system/configuration.nix). This is useful in case you