diff --git a/bntseq.c b/bntseq.c
index b63dff4..403d088 100644
--- a/bntseq.c
+++ b/bntseq.c
@@ -403,6 +403,10 @@ uint8_t *bns_fetch_seq(const bntseq_t *bns, const uint8_t *pac, int64_t *beg, in
 	*beg = *beg > far_beg? *beg : far_beg;
 	*end = *end < far_end? *end : far_end;
 	seq = bns_get_seq(bns->l_pac, pac, *beg, *end, &len);
+	if (seq == 0 || *end - *beg != len) {
+		fprintf(stderr, "[E::%s] begin=%ld, mid=%ld, end=%ld, len=%ld, seq=%p, rid=%d, far_beg=%ld, far_end=%ld\n",
+				__func__, (long)*beg, (long)mid, (long)*end, (long)len, seq, *rid, (long)far_beg, (long)far_end);
+	}
 	assert(seq && *end - *beg == len); // assertion failure should never happen
 	return seq;
 }