diff --git a/firmware/common/unicode.c b/firmware/common/unicode.c
old mode 100644
new mode 100755
index 61989e5..c31c146
--- a/firmware/common/unicode.c
+++ b/firmware/common/unicode.c
@@ -313,9 +313,19 @@ unsigned long utf8length(const unsigned char *utf8)
 {
     unsigned long l = 0;
 
+    if (!*utf8)
+        return 0;
+
     while (*utf8 != 0)
         if ((*utf8++ & MASK) != COMP)
             l++;
+  
+    /*
+     * When 0x80 <= *utf8 <= 0xbf (for all bytes).
+     * It is an illegal UTF-8 string.
+     */
+    if (l == 0)
+        return 1;
 
     return l;
 }