diff --git a/src/libsodium/sodium/utils.c b/src/libsodium/sodium/utils.c
index 3fbcb1a2..d52a2865 100644
--- a/src/libsodium/sodium/utils.c
+++ b/src/libsodium/sodium/utils.c
@@ -184,8 +184,8 @@ sodium_compare(const unsigned char *b1_, const unsigned char *b2_, size_t len)
 int
 sodium_is_zero(const unsigned char *n, const size_t nlen)
 {
-    size_t        i;
-    unsigned char d = 0U;
+    size_t                 i;
+    volatile unsigned char d = 0U;
 
     for (i = 0U; i < nlen; i++) {
         d |= n[i];