diff --git a/pkgs/servers/mail/mailman/python.nix b/pkgs/servers/mail/mailman/python.nix
index 288e48d814e4..58256a961aba 100644
--- a/pkgs/servers/mail/mailman/python.nix
+++ b/pkgs/servers/mail/mailman/python.nix
@@ -1,19 +1,21 @@
-{ python3, fetchPypi }:
+{ python3, lib, overlay ? (_: _: {}) }:
 
 python3.override {
-  packageOverrides = self: super: {
-    # does not find tests
-    alembic = super.alembic.overridePythonAttrs (oldAttrs: {
-      doCheck = false;
-    });
-    # Fixes `AssertionError: database connection isn't set to UTC`
-    psycopg2 = super.psycopg2.overridePythonAttrs (a: rec {
-      version = "2.8.6";
-      src = fetchPypi {
-        inherit version;
-        inherit (a) pname;
-        sha256 = "fb23f6c71107c37fd667cb4ea363ddeb936b348bbd6449278eb92c189699f543";
-      };
-    });
-  };
+  packageOverrides = lib.composeExtensions
+    (self: super: {
+      # does not find tests
+      alembic = super.alembic.overridePythonAttrs (oldAttrs: {
+        doCheck = false;
+      });
+      # Fixes `AssertionError: database connection isn't set to UTC`
+      psycopg2 = super.psycopg2.overridePythonAttrs (a: (rec {
+        version = "2.8.6";
+        src = super.fetchPypi {
+          inherit version;
+          inherit (a) pname;
+          sha256 = "fb23f6c71107c37fd667cb4ea363ddeb936b348bbd6449278eb92c189699f543";
+        };
+      }));
+    })
+    overlay;
 }