diff --git a/lib/resty/mongol/bson.lua b/lib/resty/mongol/bson.lua
index 8abcf98..90adb71 100755
--- a/lib/resty/mongol/bson.lua
+++ b/lib/resty/mongol/bson.lua
@@ -27,6 +27,7 @@ local new_object_id = obid.new
 local object_id_mt = obid.metatable
 local binary_mt = {}
 local utc_date = {}
+local bson_null = {}
 
 
 local function read_document ( get , numerical )
@@ -68,7 +69,7 @@ local function read_document ( get , numerical )
 		elseif op == "\9" then -- UTC datetime milliseconds
 			v = le_uint_to_num ( get ( 8 ) , 1 , 8 )
 		elseif op == "\10" then -- Null
-			v = nil
+			v = bson_null
 		elseif op == "\16" then --int32
 			v = le_int_to_num ( get ( 4 ) , 1 , 8 )
         elseif op == "\17" then --int64
@@ -209,8 +210,9 @@ function to_bson(ob)
 end
 
 return {
-	from_bson = from_bson ;
-	to_bson = to_bson ;
+    from_bson = from_bson;
+    to_bson = to_bson;
+    bson_null = bson_null;
     get_bin_data = get_bin_data;
     get_utc_date = get_utc_date;
 }
diff --git a/lib/resty/mongol/colmt.lua b/lib/resty/mongol/colmt.lua
index 0777aa5..bcc985d 100755
--- a/lib/resty/mongol/colmt.lua
+++ b/lib/resty/mongol/colmt.lua
@@ -218,7 +218,9 @@ function colmethods:query(query, returnfields, numberToSkip, numberToReturn, opt
             + 2^7*( options.Partial and 1 or 0 )
     end
 
-    query = to_bson(query)
+    if type(query) ~= "string" then
+        query = to_bson(query)
+    end
     if returnfields then
         returnfields = to_bson(returnfields)
     else
diff --git a/lib/resty/mongol/init.lua b/lib/resty/mongol/init.lua
index d6e76bd..52c88d1 100755
--- a/lib/resty/mongol/init.lua
+++ b/lib/resty/mongol/init.lua
@@ -12,6 +12,8 @@ local connmethods = { }
 local connmt = { __index = connmethods }
 
 local dbmt = require ( mod_name .. ".dbmt" )
+local bson = require ( mod_name .. ".bson" )
+null = bson.bson_null
 
 function connmethods:ismaster()
     local db = self:new_db_handle("admin")