diff --git a/index.c b/index.c
index 0d8a2ae..cd8b40e 100644
--- a/index.c
+++ b/index.c
@@ -192,6 +192,7 @@ int32_t mm_idx_cal_max_occ(const mm_idx_t *mi, float f)
 	if (f <= 0.) return INT32_MAX;
 	for (i = 0; i < 1<<mi->b; ++i)
 		if (mi->B[i].h) n += kh_size((idxhash_t*)mi->B[i].h);
+	if (n == 0) return INT32_MAX;
 	a = (uint32_t*)malloc(n * 4);
 	for (i = n = 0; i < 1<<mi->b; ++i) {
 		idxhash_t *h = (idxhash_t*)mi->B[i].h;
diff --git a/misc/paftools.js b/misc/paftools.js
index e640f95..c0c4eee 100755
--- a/misc/paftools.js
+++ b/misc/paftools.js
@@ -2076,7 +2076,7 @@ function paf_mapeval(args)
 		warn("Usage: paftools.js mapeval [options] <in.paf>|<in.sam>");
 		warn("Options:");
 		warn("  -r FLOAT   mapping correct if overlap_length/union_length>FLOAT [" + ovlp_ratio + "]");
-		warn("  -Q INT     print wrong mappings with mapQ>INT [don't print]");
+		warn("  -Q INT     print wrong mappings with mapQ>=INT [don't print]");
 		warn("  -m INT     0: eval the longest aln only; 1: first aln only; 2: all primary aln [0]");
 		exit(1);
 	}