diff --git a/wots.c b/wots.c
index d9c8449..4764605 100644
--- a/wots.c
+++ b/wots.c
@@ -80,9 +80,9 @@ static void base_w(const xmss_params *params,
 
 /* Computes the WOTS+ checksum over a message (in base_w). */
 static void wots_checksum(const xmss_params *params,
-                          int *csum_base_w, const int *msg_base_w)
+                          int *csum_base_w, const unsigned int *msg_base_w)
 {
-    int csum = 0;
+    unsigned long long csum = 0;
     unsigned char csum_bytes[(params->wots_len2 * params->wots_log_w + 7) / 8];
     unsigned int i;
 
@@ -100,7 +100,7 @@ static void wots_checksum(const xmss_params *params,
 
 /* Takes a message and derives the matching chain lengths. */
 static void chain_lengths(const xmss_params *params,
-                          int *lengths, const unsigned char *msg)
+                          unsigned int *lengths, const unsigned char *msg)
 {
     base_w(params, lengths, params->wots_len1, msg);
     wots_checksum(params, lengths + params->wots_len1, lengths);
@@ -139,7 +139,7 @@ void wots_sign(const xmss_params *params,
                const unsigned char *seed, const unsigned char *pub_seed,
                uint32_t addr[8])
 {
-    int lengths[params->wots_len];
+    unsigned int lengths[params->wots_len];
     uint32_t i;
 
     chain_lengths(params, lengths, msg);
@@ -163,7 +163,7 @@ void wots_pk_from_sig(const xmss_params *params, unsigned char *pk,
                       const unsigned char *sig, const unsigned char *msg,
                       const unsigned char *pub_seed, uint32_t addr[8])
 {
-    int lengths[params->wots_len];
+    unsigned int lengths[params->wots_len];
     uint32_t i;
 
     chain_lengths(params, lengths, msg);