diff --git a/nixos/modules/overlays.nix b/nixos/modules/overlays.nix
index 779384ab6914ea4858970f7190780a7543702913..a644e630e4fd01da411cbc4200ea3501a1965550 100644
--- a/nixos/modules/overlays.nix
+++ b/nixos/modules/overlays.nix
@@ -34,6 +34,10 @@ self: super: {
   # the value from the previously overlay, not from the fixed point.  This is
   # important because this override never converges.
   python27 = super.python27.override (old: {
-    packageOverrides = super.lib.composeExtensions old.packageOverrides pythonTwistedOverride;
+    packageOverrides =
+      if old ? packageOverrides then
+        super.lib.composeExtensions old.packageOverrides pythonTwistedOverride
+      else
+        pythonTwistedOverride;
   });
 }