diff --git a/pkgs/development/compilers/circt/default.nix b/pkgs/development/compilers/circt/default.nix
index ce06782eb12..a8e36cbc785 100644
--- a/pkgs/development/compilers/circt/default.nix
+++ b/pkgs/development/compilers/circt/default.nix
@@ -49,7 +49,8 @@ stdenv.mkDerivation rec {
   #    https://github.com/NixOS/nixpkgs/issues/214945 discusses this issue.
   #
   # As a temporary fix, we disabled these tests when using clang stdenv
-  LIT_FILTER_OUT = lib.optionalString stdenv.cc.isClang "CIRCT :: Target/ExportSystemC/.*\.mlir";
+  # cannot use lib.optionalString as it creates an empty string, disabling all tests
+  LIT_FILTER_OUT = if stdenv.cc.isClang then "CIRCT :: Target/ExportSystemC/.*\.mlir" else null;
 
   preConfigure = ''
     substituteInPlace test/circt-reduce/test/annotation-remover.mlir --replace "/usr/bin/env" "${coreutils}/bin/env"