diff --git a/cmp11sht.c b/cmp11sht.c
index a76cd39..33cadca 100644
--- a/cmp11sht.c
+++ b/cmp11sht.c
@@ -169,6 +169,8 @@ int sequal(char *a, char *b, float thr, float *ratio, char *s1, char *s2)
 {
     int i;
 
+    if(!a || !b || !s1 || !s2 || !ratio) return 3; /* error sentinel */
+
     // remove accents
     asciify(a, s1);
     asciify(b, s2);