diff --git a/index.c b/index.c
index cfdef19..d7ddc6e 100644
--- a/index.c
+++ b/index.c
@@ -48,10 +48,12 @@ void mm_idx_destroy(mm_idx_t *mi)
 	uint32_t i;
 	if (mi == 0) return;
 	if (mi->h) kh_destroy(str, (khash_t(str)*)mi->h);
-	for (i = 0; i < 1U<<mi->b; ++i) {
-		free(mi->B[i].p);
-		free(mi->B[i].a.a);
-		kh_destroy(idx, (idxhash_t*)mi->B[i].h);
+	if (mi->B) {
+		for (i = 0; i < 1U<<mi->b; ++i) {
+			free(mi->B[i].p);
+			free(mi->B[i].a.a);
+			kh_destroy(idx, (idxhash_t*)mi->B[i].h);
+		}
 	}
 	if (!mi->km) {
 		for (i = 0; i < mi->n_seq; ++i)
diff --git a/main.c b/main.c
index ee12823..00324ed 100644
--- a/main.c
+++ b/main.c
@@ -10,7 +10,7 @@
 #include "getopt.h"
 #endif
 
-#define MM_VERSION "2.11-r814-dirty"
+#define MM_VERSION "2.11-r815-dirty"
 
 #ifdef __linux__
 #include <sys/resource.h>
@@ -330,7 +330,7 @@ int main(int argc, char *argv[])
 			} else {
 				mm_write_sam_hdr(0, rg, MM_VERSION, argc, argv);
 				if (opt.split_prefix == 0 && mm_verbose >= 2)
-					fprintf(stderr, "[WARNING]\033[1;31m For a multi-part index, no @SQ lines will be outputted.\033[0m\n");
+					fprintf(stderr, "[WARNING]\033[1;31m For a multi-part index, no @SQ lines will be outputted. Please use --split-prefix.\033[0m\n");
 			}
 		}
 		if (mm_verbose >= 3)
diff --git a/map.c b/map.c
index 1a6ff1a..ba04d32 100644
--- a/map.c
+++ b/map.c
@@ -659,6 +659,7 @@ int mm_split_merge(int n_segs, const char **fn, const mm_mapopt_t *opt, int n_sp
 {
 	int i;
 	pipeline_t pl;
+	mm_idx_t *mi;
 	if (n_segs < 1 || n_split_idx < 1) return -1;
 	memset(&pl, 0, sizeof(pipeline_t));
 	pl.n_fp = n_segs;
@@ -670,7 +671,7 @@ int mm_split_merge(int n_segs, const char **fn, const mm_mapopt_t *opt, int n_sp
 	pl.n_parts = n_split_idx;
 	pl.fp_parts  = CALLOC(FILE*, pl.n_parts);
 	pl.rid_shift = CALLOC(uint32_t, pl.n_parts);
-	pl.mi = mm_split_merge_prep(opt->split_prefix, n_split_idx, pl.fp_parts, pl.rid_shift);
+	pl.mi = mi = mm_split_merge_prep(opt->split_prefix, n_split_idx, pl.fp_parts, pl.rid_shift);
 	if (pl.mi == 0) {
 		free(pl.fp_parts);
 		free(pl.rid_shift);
@@ -687,6 +688,7 @@ int mm_split_merge(int n_segs, const char **fn, const mm_mapopt_t *opt, int n_sp
 	kt_pipeline(2, worker_pipeline, &pl, 3);
 
 	free(pl.str.s);
+	mm_idx_destroy(mi);
 	free(pl.rid_shift);
 	for (i = 0; i < n_split_idx; ++i)
 		fclose(pl.fp_parts[i]);
diff --git a/minimap2.1 b/minimap2.1
index 0d240d1..ced6c8c 100644
--- a/minimap2.1
+++ b/minimap2.1
@@ -239,7 +239,7 @@ Disable the long gap patching heuristic. When this option is applied, the
 maximum alignment gap is mostly controlled by
 .BR -r .
 .TP
-.B --lj-min-ratio \ FLOAT
+.BI --lj-min-ratio \ FLOAT
 Fraction of query sequence length required to bridge a long gap [0.5]. A
 smaller value helps to recover longer gaps, at the cost of more false gaps.
 .TP
@@ -252,6 +252,9 @@ applies a second round of chaining with a higher minimizer occurrence threshold
 if no good chain is found. In addition, minimap2 attempts to patch gaps between
 seeds with ungapped alignment.
 .TP
+.BI --split-prefix \ STR
+Prefix to create temporary files. Typically used for a multi-part index.
+.TP
 .BR --frag = no | yes
 Whether to enable the fragment mode [no]
 .TP