diff --git a/mkfs/mkfs.c b/mkfs/mkfs.c
index 1ec326b..6f4bd9b 100644
--- a/mkfs/mkfs.c
+++ b/mkfs/mkfs.c
@@ -147,6 +147,8 @@ main(int argc, char *argv[])
     if(shortname[0] == '_')
       shortname += 1;
 
+    assert(strlen(shortname) <= DIRSIZ);
+    
     inum = ialloc(T_FILE);
 
     bzero(&de, sizeof(de));