diff --git a/NEWS b/NEWS
index 42e7c79..86f4114 100644
--- a/NEWS
+++ b/NEWS
@@ -2,8 +2,7 @@ Release 0.7.4 (23 April, 2013)
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 This is a bugfix release. Most of bugs are considered to be minor which only
- very rarely.
- ccur
+occur very rarely.
 
  * Bugfix: wrong CIGAR when a query sequence bridges three or more target
    sequences. This only happens when aligning reads to short assembly contigs.
diff --git a/bwamem.c b/bwamem.c
index 8c2ba9c..9f3aa9b 100644
--- a/bwamem.c
+++ b/bwamem.c
@@ -951,7 +951,7 @@ static void *worker2(void *data)
 	return 0;
 }
 
-void mem_process_seqs(const mem_opt_t *opt, const bwt_t *bwt, const bntseq_t *bns, const uint8_t *pac, int n, bseq1_t *seqs)
+void mem_process_seqs(const mem_opt_t *opt, const bwt_t *bwt, const bntseq_t *bns, const uint8_t *pac, int n, bseq1_t *seqs, const mem_pestat_t *pes0)
 {
 	int i;
 	worker_t *w;
@@ -967,29 +967,30 @@ void mem_process_seqs(const mem_opt_t *opt, const bwt_t *bwt, const bntseq_t *bn
 		p->seqs = seqs; p->regs = regs;
 		p->pes = &pes[0];
 	}
+
 #ifdef HAVE_PTHREAD
 	if (opt->n_threads == 1) {
+#endif
 		worker1(w);
-		if (opt->flag&MEM_F_PE) mem_pestat(opt, bns->l_pac, n, regs, pes);
+		if (opt->flag&MEM_F_PE) { // paired-end mode
+			if (pes0) memcpy(pes, pes0, 4 * sizeof(mem_pestat_t)); // if pes0 != NULL, set the insert-size distribution as pes0
+			else mem_pestat(opt, bns->l_pac, n, regs, pes); // otherwise, infer the insert size distribution from data
+		}
 		worker2(w);
+#ifdef HAVE_PTHREAD
 	} else {
 		pthread_t *tid;
 		tid = (pthread_t*)xcalloc(opt->n_threads, sizeof(pthread_t));
 		for (i = 0; i < opt->n_threads; ++i) pthread_create(&tid[i], 0, worker1, &w[i]);
 		for (i = 0; i < opt->n_threads; ++i) pthread_join(tid[i], 0);
-		if (opt->flag&MEM_F_PE) mem_pestat(opt, bns->l_pac, n, regs, pes);
+		if (opt->flag&MEM_F_PE) {
+			if (pes0) memcpy(pes, pes0, 4 * sizeof(mem_pestat_t));
+			else mem_pestat(opt, bns->l_pac, n, regs, pes);
+		}
 		for (i = 0; i < opt->n_threads; ++i) pthread_create(&tid[i], 0, worker2, &w[i]);
 		for (i = 0; i < opt->n_threads; ++i) pthread_join(tid[i], 0);
 		free(tid);
 	}
-#else
-	worker1(w);
-	if (opt->flag&MEM_F_PE) mem_pestat(opt, bns->l_pac, n, regs, pes);
-	worker2(w);
 #endif
-	for (i = 0; i < n; ++i) {
-		err_fputs(seqs[i].sam, stdout);
-		free(seqs[i].name); free(seqs[i].comment); free(seqs[i].seq); free(seqs[i].qual); free(seqs[i].sam);
-	}
 	free(regs); free(w);
 }
diff --git a/bwamem.h b/bwamem.h
index 40f47f8..76be8e3 100644
--- a/bwamem.h
+++ b/bwamem.h
@@ -57,8 +57,9 @@ typedef struct {
 typedef struct { size_t n, m; mem_alnreg_t *a; } mem_alnreg_v;
 
 typedef struct {
-	int low, high, failed;
-	double avg, std;
+	int low, high;   // lower and upper bounds within which a read pair is considered to be properly paired
+	int failed;      // non-zero if the orientation is not supported by sufficient data
+	double avg, std; // mean and stddev of the insert size distribution
 } mem_pestat_t;
 
 typedef struct { // This struct is only used for the convenience of API.
@@ -103,8 +104,10 @@ extern "C" {
 	 * @param pac    2-bit encoded reference
 	 * @param n      number of query sequences
 	 * @param seqs   query sequences; $seqs[i].seq/sam to be modified after the call
+	 * @param pes0   insert-size info; if NULL, infer from data; if not NULL, it should be an array with 4 elements,
+	 *               corresponding to each FF, FR, RF and RR orientation. See mem_pestat() for more info.
 	 */
-	void mem_process_seqs(const mem_opt_t *opt, const bwt_t *bwt, const bntseq_t *bns, const uint8_t *pac, int n, bseq1_t *seqs);
+	void mem_process_seqs(const mem_opt_t *opt, const bwt_t *bwt, const bntseq_t *bns, const uint8_t *pac, int n, bseq1_t *seqs, const mem_pestat_t *pes0);
 
 	/**
 	 * Find the aligned regions for one query sequence
diff --git a/bwase.c b/bwase.c
index 85efcbd..41ab175 100644
--- a/bwase.c
+++ b/bwase.c
@@ -181,6 +181,7 @@ bwa_cigar_t *bwa_refine_gapped_core(bwtint_t l_pac, const ubyte_t *pacseq, int l
 		rb = re - tle; rlen = tle;
 		seq_reverse(len, seq, 0);
 		seq_reverse(rlen, rseq, 0);
+		if (rlen == 0) goto refine_gapped_err;
 		ksw_global(qle, &seq[len-qle], rlen, rseq, 5, mat, 5, 1, SW_BW, n_cigar, &cigar32);
 		if (qle < len) { // write soft clip
 			cigar = xrealloc(cigar, (*n_cigar + 1) * 4);
@@ -195,6 +196,7 @@ bwa_cigar_t *bwa_refine_gapped_core(bwtint_t l_pac, const ubyte_t *pacseq, int l
 		ksw_extend(len, seq, rlen, rseq, 5, mat, 5, 1, SW_BW, 0, -1, len<<1, &qle, &tle, &gtle, &gscore, 0);
 		if (gscore > 0) tle = gtle, qle = len;
 		re = rb + tle; rlen = tle;
+		if (rlen == 0) goto refine_gapped_err;
 		ksw_global(qle, seq, rlen, rseq, 5, mat, 5, 1, SW_BW, n_cigar, &cigar32); // right extension
 		if (qle < len) {
 			cigar = xrealloc(cigar, (*n_cigar + 1) * 4);
@@ -209,6 +211,11 @@ bwa_cigar_t *bwa_refine_gapped_core(bwtint_t l_pac, const ubyte_t *pacseq, int l
 		cigar[k] = __cigar_create((cigar32[k]&0xf), (cigar32[k]>>4));
 	free(rseq);
 	return cigar;
+
+refine_gapped_err:
+	free(rseq);
+	*n_cigar = 0;
+	return 0;
 }
 
 char *bwa_cal_md1(int n_cigar, bwa_cigar_t *cigar, int len, bwtint_t pos, ubyte_t *seq,
@@ -320,6 +327,7 @@ void bwa_refine_gapped(const bntseq_t *bns, int n_seqs, bwa_seq_t *seqs, ubyte_t
 		}
 		if (s->type == BWA_TYPE_NO_MATCH || s->type == BWA_TYPE_MATESW || s->n_gapo == 0) continue;
 		s->cigar = bwa_refine_gapped_core(bns->l_pac, pacseq, s->len, s->strand? s->rseq : s->seq, &s->pos, &s->n_cigar, s->strand);
+		if (s->cigar == 0) s->type = BWA_TYPE_NO_MATCH;
 	}
 	// generate MD tag
 	str = (kstring_t*)xcalloc(1, sizeof(kstring_t));
@@ -475,6 +483,7 @@ void bwa_print_sam1(const bntseq_t *bns, bwa_seq_t *p, const bwa_seq_t *mate, in
 				for (i = 0; i < p->n_multi; ++i) {
 					bwt_multi1_t *q = p->multi + i;
 					int k;
+					if (q->cigar == 0) continue;
 					j = pos_end_multi(q, p->len) - q->pos;
 					nn = bns_cnt_ambi(bns, q->pos, j, &seqid);
 					err_printf("%s,%c%d,", bns->anns[seqid].name, q->strand? '-' : '+',
diff --git a/fastmap.c b/fastmap.c
index 7ff73cb..d0651f9 100644
--- a/fastmap.c
+++ b/fastmap.c
@@ -91,9 +91,12 @@ int main_mem(int argc, char *argv[])
 
 	bwa_fill_scmat(opt->a, opt->b, opt->mat);
 	if ((idx = bwa_idx_load(argv[optind], BWA_IDX_ALL)) == 0) return 1; // FIXME: memory leak
-	bwa_print_sam_hdr(idx->bns, rg_line);
 
 	ko = kopen(argv[optind + 1], &fd);
+	if (ko == 0) {
+		if (bwa_verbose >= 1) fprintf(stderr, "[E::%s] fail to open file `%s'.\n", __func__, argv[optind + 1]);
+		return 1;
+	}
 	fp = gzdopen(fd, "r");
 	ks = kseq_init(fp);
 	if (optind + 2 < argc) {
@@ -102,11 +105,16 @@ int main_mem(int argc, char *argv[])
 				fprintf(stderr, "[W::%s] when '-p' is in use, the second query file will be ignored.\n", __func__);
 		} else {
 			ko2 = kopen(argv[optind + 2], &fd2);
+			if (ko2 == 0) {
+				if (bwa_verbose >= 1) fprintf(stderr, "[E::%s] fail to open file `%s'.\n", __func__, argv[optind + 2]);
+				return 1;
+			}
 			fp2 = gzdopen(fd2, "r");
 			ks2 = kseq_init(fp2);
 			opt->flag |= MEM_F_PE;
 		}
 	}
+	bwa_print_sam_hdr(idx->bns, rg_line);
 	while ((seqs = bseq_read(opt->chunk_size * opt->n_threads, &n, ks, ks2)) != 0) {
 		int64_t size = 0;
 		if ((opt->flag & MEM_F_PE) && (n&1) == 1) {
@@ -121,7 +129,11 @@ int main_mem(int argc, char *argv[])
 		for (i = 0; i < n; ++i) size += seqs[i].l_seq;
 		if (bwa_verbose >= 3)
 			fprintf(stderr, "[M::%s] read %d sequences (%ld bp)...\n", __func__, n, (long)size);
-		mem_process_seqs(opt, idx->bwt, idx->bns, idx->pac, n, seqs);
+		mem_process_seqs(opt, idx->bwt, idx->bns, idx->pac, n, seqs, 0);
+		for (i = 0; i < n; ++i) {
+			err_fputs(seqs[i].sam, stdout);
+			free(seqs[i].name); free(seqs[i].comment); free(seqs[i].seq); free(seqs[i].qual); free(seqs[i].sam);
+		}
 		free(seqs);
 	}
 
diff --git a/kopen.c b/kopen.c
index c1c43a8..82f2812 100644
--- a/kopen.c
+++ b/kopen.c
@@ -319,14 +319,14 @@ void *kopen(const char *fn, int *_fd)
 #else
 			*_fd = open(fn, O_RDONLY);
 #endif
-			if (*_fd) {
+			if (*_fd >= 0) {
 				aux = xcalloc(1, sizeof(koaux_t));
 				aux->type = KO_FILE;
 				aux->fd = *_fd;
 			}
 		}
 	}
-	*_fd = aux->fd;
+	if (aux) *_fd = aux->fd;
 	return aux;
 }
 
diff --git a/main.c b/main.c
index 502af35..cc1a4b4 100644
--- a/main.c
+++ b/main.c
@@ -3,7 +3,7 @@
 #include "utils.h"
 
 #ifndef PACKAGE_VERSION
-#define PACKAGE_VERSION "0.7.4-r385"
+#define PACKAGE_VERSION "0.7.4-r389-beta"
 #endif
 
 int bwa_fa2pac(int argc, char *argv[]);
@@ -92,5 +92,5 @@ int main(int argc, char *argv[])
 			fprintf(stderr, " %s", argv[i]);
 		fprintf(stderr, "\n[%s] Real time: %.3f sec; CPU: %.3f sec\n", __func__, realtime() - t_real, cputime());
 	}
-	return 0;
+	return ret;
 }