diff --git a/minimap.h b/minimap.h
index 6f571fa..3223a8d 100644
--- a/minimap.h
+++ b/minimap.h
@@ -5,7 +5,7 @@
 #include <stdio.h>
 #include <sys/types.h>
 
-#define MM_VERSION "2.28-r1240-dirty"
+#define MM_VERSION "2.28-r1241-dirty"
 
 #define MM_F_NO_DIAG       (0x001LL) // no exact diagonal hit
 #define MM_F_NO_DUAL       (0x002LL) // skip pairs where query name is lexicographically larger than target name
diff --git a/minimap2.1 b/minimap2.1
index 6943b8e..59bff44 100644
--- a/minimap2.1
+++ b/minimap2.1
@@ -414,7 +414,7 @@ no attempt to match GT-AG [n]
 Score bonus when alignment extends to the end of the query sequence [0].
 .TP
 .BI --score-N \ INT
-Score of a mismatch involving ambiguous bases [1].
+Penalty of a mismatch involving ambiguous bases [1].
 .TP
 .BI --pe-ind-chain
 For paired-end short reads, perform chaining for each end independently.
diff --git a/options.c b/options.c
index 2a6032c..0fb6af7 100644
--- a/options.c
+++ b/options.c
@@ -249,6 +249,11 @@ int mm_check_opt(const mm_idxopt_t *io, const mm_mapopt_t *mo)
 			fprintf(stderr, "[ERROR]\033[1;31m scoring system violating ({-O}+{-E})+({-O2}+{-E2}) <= 127\033[0m\n");
 		return -1;
 	}
+	if (mo->sc_ambi < 0 || mo->sc_ambi >= mo->b) {
+		if (mm_verbose >= 1)
+			fprintf(stderr, "[ERROR]\033[1;31m --score-N should be within [0,{-B})\033[0m\n");
+		return -1;
+	}
 	if (mo->zdrop < mo->zdrop_inv) {
 		if (mm_verbose >= 1)
 			fprintf(stderr, "[ERROR]\033[1;31m Z-drop should not be less than inversion-Z-drop\033[0m\n");