diff --git a/src/libsodium/crypto_scalarmult/curve25519/scalarmult_curve25519.c b/src/libsodium/crypto_scalarmult/curve25519/scalarmult_curve25519.c
index 13eda805..0c75b019 100644
--- a/src/libsodium/crypto_scalarmult/curve25519/scalarmult_curve25519.c
+++ b/src/libsodium/crypto_scalarmult/curve25519/scalarmult_curve25519.c
@@ -20,8 +20,8 @@ int
 crypto_scalarmult_curve25519(unsigned char *q, const unsigned char *n,
                              const unsigned char *p)
 {
-    size_t        i;
-    unsigned char d = 0;
+    size_t                 i;
+    volatile unsigned char d = 0;
 
     if (implementation->mult(q, n, p) != 0) {
         return -1; /* LCOV_EXCL_LINE */