--- a/hglib.lua	Mon Oct 24 14:05:51 2016 +0800
+++ b/hglib.lua	Mon Oct 24 14:07:22 2016 +0800
@@ -157,7 +157,7 @@
 	local o = ''
 	local e = ''
 	local d = ''
-	input = input or ''
+	input = tostring(input or '')
 	local function write_input(length)
 		write_block(self.wh, input:sub(1, length))
 		self.wh:flush()