diff --git a/nix/default.nix b/nix/default.nix
index c60e08b12529e01b22cd5678a202b948b3b6d01a..652b5136731b658dfdcfde3d6ee91f6069665d77 100644
--- a/nix/default.nix
+++ b/nix/default.nix
@@ -20,7 +20,7 @@ let frama_clang_build =
      preFramaCTests = ''
        echo CONFIGURING Why3 for Frama_Clang.
        export HOME=$(mktemp -d)
-       why3 config --detect
+       why3 config detect
      '';
    });
 in