diff --git a/daemon/crypto.c b/daemon/crypto.c
index 0758c0e7a..3b71591db 100644
--- a/daemon/crypto.c
+++ b/daemon/crypto.c
@@ -175,7 +175,7 @@ static void prf_n(str *out, char *key, char *x) {
 	memcpy(iv, x, 14);
 	/* iv[14] = iv[15] = 0;   := x << 16 */
 	ZERO(in); /* outputs the key stream */
-	str_init_len(&in_s, in, 16);
+	str_init_len(&in_s, in, sizeof(in));
 	aes_ctr_128(o, &in_s, key, iv);
 
 	memcpy(out->s, o, out->len);