diff --git a/flake.nix b/flake.nix
index fda7173..308e5b6 100644
--- a/flake.nix
+++ b/flake.nix
@@ -113,7 +113,7 @@
                                            # interface-file overflow in GHC 9.10.3) into two halves.
                                            # All awk/printf: no external file deps, no git-tracking required.
                                            # -O0 keeps .hi files small (strips unfoldings/specialisations).
-                                           configureFlags = (old.configureFlags or "") + " --ghc-option=-O0";
+                                           configureFlags = (old.configureFlags or []) ++ [ "--ghc-option=-O0" ];
                                            postUnpack = (old.postUnpack or "") + ''
                                                _types="$sourceRoot/build/Generated/Types.hs"