diff --git a/pkgs/development/python-modules/pytest-xdist/default.nix b/pkgs/development/python-modules/pytest-xdist/default.nix
index 555c840311442..972d0bdf23c71 100644
--- a/pkgs/development/python-modules/pytest-xdist/default.nix
+++ b/pkgs/development/python-modules/pytest-xdist/default.nix
@@ -45,10 +45,10 @@ buildPythonPackage rec {
     setproctitle = [ setproctitle ];
   };
 
-  pytestFlagsArray = [
-    # pytest can already use xdist at this point
-    "--numprocesses=$NIX_BUILD_CORES"
-  ];
+  # pytest can already use xdist at this point
+  preCheck = ''
+    appendToVar pytestFlagsArray "--numprocesses=$NIX_BUILD_CORES"
+  '';
 
   # access file system
   disabledTests = [