diff --git a/xmss_fast.c b/xmss_fast.c
index 9b436c9..0a28935 100644
--- a/xmss_fast.c
+++ b/xmss_fast.c
@@ -948,8 +948,10 @@ int xmssmt_sign(unsigned char *sk, bds_state *states, unsigned char *wots_sigs,
 
   SET_LAYER_ADDRESS(addr, 0);
   SET_TREE_ADDRESS(addr, (idx_tree + 1));
-  // mandatory update for NEXT_0 (does not count towards h-k/2)
-  bds_state_update(&states[params->d], sk_seed, &(params->xmss_par), pub_seed, addr);
+  // mandatory update for NEXT_0 (does not count towards h-k/2) if NEXT_0 exists
+  if ((1 + idx_tree) * (1 << tree_h) + idx_leaf < (1UL << h)) {
+    bds_state_update(&states[params->d], sk_seed, &(params->xmss_par), pub_seed, addr);
+  }
 
   for (i = 0; i < params->d; i++) {
     // check if we're not at the end of a tree