diff --git a/HISTORY.md b/HISTORY.md
index 60609957..aca45755 100644
--- a/HISTORY.md
+++ b/HISTORY.md
@@ -13,6 +13,7 @@ Dev version
 * add DEBUG command exit (send a message to lua service by DEBUG)
 * add DEBUG command run (debug_console command inject)
 * bugfix : socketchannel connect once
+* bugfix : mongo driver
 
 v0.5.2 (2014-8-11)
 -----------
diff --git a/lualib-src/lua-mongo.c b/lualib-src/lua-mongo.c
index 855cc385..af63d0b0 100644
--- a/lualib-src/lua-mongo.c
+++ b/lualib-src/lua-mongo.c
@@ -507,8 +507,8 @@ op_insert(lua_State *L) {
 		for (i=1;i<=s;i++) {
 			lua_rawgeti(L,3,i);
 			document doc = lua_touserdata(L,-1);
+			lua_pop(L,1);	// must call lua_pop before luaL_addlstring, because addlstring may change stack top
 			luaL_addlstring(&b, (const char *)doc, get_length(doc));
-			lua_pop(L,1);
 		}
 	}