diff --git a/ref/fors.c b/ref/fors.c
index a4f2a199..3599e5b6 100644
--- a/ref/fors.c
+++ b/ref/fors.c
@@ -56,7 +56,7 @@ static void message_to_indices(uint32_t *indices, const unsigned char *m)
     for (i = 0; i < SPX_FORS_TREES; i++) {
         indices[i] = 0;
         for (j = 0; j < SPX_FORS_HEIGHT; j++) {
-            indices[i] ^= ((m[offset >> 3] >> (offset & 0x7)) & 0x1) << j;
+            indices[i] ^= ((m[offset >> 3] >> (~offset & 0x7)) & 0x1) << j;
             offset++;
         }
     }