diff --git a/lib11sht.c b/lib11sht.c
index 1f37528..5d4f277 100644
--- a/lib11sht.c
+++ b/lib11sht.c
@@ -82,8 +82,10 @@ static float shit11(char *s1, char *s2)
     len1 = strlen(s1);
     len2 = strlen(s2);
 
+    if(len1 == 0 && len2 == 0)
+        return 1.0;                 /* both empty == identical */
     if(len1 == 0 || len2 == 0)
-        return 0.0;
+        return 0.0;                 /* one empty == no similarity */
 
     int d[len1+1][len2+1];