diff --git a/src/mbox.lua b/src/mbox.lua
index 7724ae2..ed9e781 100644
--- a/src/mbox.lua
+++ b/src/mbox.lua
@@ -61,7 +61,7 @@ function _M.parse_from(from)
 end
 
 function _M.split_mbox(mbox_s)
-    mbox = {}
+    local mbox = {}
     mbox_s = string.gsub(mbox_s, "\r\n", "\n") .."\n\nFrom \n"
     local nj, i, j = 1, 1, 1
     while 1 do