diff --git a/main.c b/main.c
index 6ce6f9e..0768aa3 100644
--- a/main.c
+++ b/main.c
@@ -6,7 +6,7 @@
 #include "mmpriv.h"
 #include "getopt.h"
 
-#define MM_VERSION "2.7-r657-dirty"
+#define MM_VERSION "2.7-r658-dirty"
 
 #ifdef __linux__
 #include <sys/resource.h>
diff --git a/map.c b/map.c
index eac2600..11495b4 100644
--- a/map.c
+++ b/map.c
@@ -117,6 +117,13 @@ int mm_set_opt(const char *preset, mm_idxopt_t *io, mm_mapopt_t *mo)
 
 int mm_check_opt(const mm_idxopt_t *io, const mm_mapopt_t *mo)
 {
+	if (mo->best_n < 0) {
+		if (mm_verbose >= 1)
+			fprintf(stderr, "[ERROR]\033[1;31m -N must be no less than 0\033[0m\n");
+		return -4;
+	}
+	if (mo->best_n == 0 && mm_verbose >= 2)
+		fprintf(stderr, "[WARNING]\033[1;31m '-N 0' reduces mapping accuracy. Please use '--secondary=no' instead.\033[0m\n");
 	if (mo->pri_ratio < 0.0f || mo->pri_ratio > 1.0f) {
 		if (mm_verbose >= 1)
 			fprintf(stderr, "[ERROR]\033[1;31m -p must be within 0 and 1 (including 0 and 1)\033[0m\n");