Lua script coverts empty array to object

Lua script converts empty array as an object. How to avoid conversion.

test.lua

local json_str = '{\"items\":[],\"properties\":{}}'return cjson.encode(cjson.decode(json_str))

Output

redis-cli --eval test.lua

"{\"items\":{},\"properties\":{}}"

items are an array [] but the output is an object {}