diff --git a/pkgs/development/interpreters/python/mk-python-derivation.nix b/pkgs/development/interpreters/python/mk-python-derivation.nix
index 0c1a1cf33ddb..aa304ecf2eed 100644
--- a/pkgs/development/interpreters/python/mk-python-derivation.nix
+++ b/pkgs/development/interpreters/python/mk-python-derivation.nix
@@ -205,10 +205,9 @@ in
   ...
 }@attrs:
 
-assert (pyproject != null) -> (format == null);
-
 let
   format' =
+    assert (pyproject != null) -> (format == null);
     if pyproject != null then
       if pyproject then "pyproject" else "other"
     else if format != null then