diff --git a/src/crypto.c b/src/crypto.c
index 3128b2a5..7235e3c2 100644
--- a/src/crypto.c
+++ b/src/crypto.c
@@ -331,7 +331,7 @@ bool aes_siv_encrypt(const void *key, size_t key_len, const void *in,
 	struct iovec iov[num_ad + 1];
 	uint8_t v[16];
 
-	if (ad)
+	if (ad && num_ad)
 		memcpy(iov, ad, sizeof(struct iovec) * num_ad);
 
 	iov[num_ad].iov_base = (void *)in;