diff --git a/driver-ztex.c b/driver-ztex.c
index d126580a..2b8d82e8 100644
--- a/driver-ztex.c
+++ b/driver-ztex.c
@@ -282,7 +282,7 @@ static int64_t ztex_scanhash(struct thr_info *thr, struct work *work,
 #endif
 			if (nonce > noncecnt)
 				noncecnt = nonce;
-			if (((nonce & 0x7fffffff) >> 4) < ((lastnonce[i] & 0x7fffffff) >> 4)) {
+			if (((0xffffffff - nonce) < (nonce - lastnonce[i])) || nonce < lastnonce[i]) {
 				applog(LOG_DEBUG, "%s: overflow nonce=%0.8x lastnonce=%0.8x", ztex->repr, nonce, lastnonce[i]);
 				overflow = true;
 			} else