diff --git a/Utilities/cmlibarchive/libarchive/archive_blake2sp_ref.c b/Utilities/cmlibarchive/libarchive/archive_blake2sp_ref.c
index b913a4db65..4f9b2d9949 100644
--- a/Utilities/cmlibarchive/libarchive/archive_blake2sp_ref.c
+++ b/Utilities/cmlibarchive/libarchive/archive_blake2sp_ref.c
@@ -28,6 +28,9 @@
 
 #define PARALLELISM_DEGREE 8
 
+/* Remove system-defined preprocessor defintions that conflict with us.  */
+#undef FS
+
 /*
   blake2sp_init_param defaults to setting the expecting output length
   from the digest_length parameter block field.