diff --git a/nixos/modules/services/databases/postgresql.nix b/nixos/modules/services/databases/postgresql.nix
index 4cff2818f3b..bae088c6610 100644
--- a/nixos/modules/services/databases/postgresql.nix
+++ b/nixos/modules/services/databases/postgresql.nix
@@ -154,6 +154,12 @@ in
 
   config = mkIf config.services.postgresql.enable {
 
+    services.postgresql.package =
+      # Note: when changing the default, make it conditional on
+      # ‘system.stateVersion’ to maintain compatibility with existing
+      # systems!
+      mkDefault pkgs.postgresql94;
+
     services.postgresql.authentication = mkAfter
       ''
         # Generated file; do not edit!