--- a/modules/libmar/src/mar_read.c
+++ b/modules/libmar/src/mar_read.c
@@ -91,16 +91,20 @@ static int mar_consume_index(MarFile *ma
   name = *buf;
   /* find namelen; must take care not to read beyond buf_end */
   while (**buf) {
     if (*buf == buf_end)
       return -1;
     ++(*buf);
   }
   namelen = (*buf - name);
+  /* must ensure that namelen is valid */
+  if (namelen < 0) {
+    return -1;
+  }
   /* consume null byte */
   if (*buf == buf_end)
     return -1;
   ++(*buf);
 
   return mar_insert_item(mar, name, namelen, offset, length, flags);
 }