bad argument #1 to 'rawget' (table expected, got string)

[fqtrnt]

I am new to lua.
A case, i was built a nginx + luajit +redis2.
I was used json.lua and got follow errer message:

2021/02/07 08:42:02 [error] 37#37: *11 failed to run body_filter_by_lua*: /etc/nginx/redis.lua:238: bad argument #1 to 'rawget' (table expected, got string)
stack traceback:
	[C]: in function 'rawget'
	/etc/nginx/redis.lua:238: in function 'decode'
	/etc/nginx/ueditor/ueditor_upload_file_rewrite_response.lua:22: in function </etc/nginx/ueditor/ueditor_upload_file_rewrite_response.lua:1> 

code fragments

if eof then
    local whole = table.concat(ngx.ctx.buffered)
    local ret_body = whole
    if whole then
        ngx.ctx.buffered = nil
        local body_json = json.decode(whole)     #################### line number 22 ####################
        local ret_table = {}
        if body_json then
            ret_table['original'] = body_json['originalFilename']
            ret_table['size'] = body_json['size']
            ret_table['state'] = 'SUCCESS'
            ret_table['title'] = body_json['originalFilename']:match("(.+)%.%w+$")
            ret_table['type'] = body_json['file']:match(".+(%.%w+)$")
            local imageUrl = body_json['thumbnail']
            if imageUrl and imageUrl ~= "" then
                ret_table['url'] = imageUrl
            else
                ret_table['url'] = body_json["file"]
            end
        else
            ret_table['state'] = 'Error: upload file failed!'
        end
        ret_body = json.encode(ret_table)
    end
    ngx.arg[1] = ret_body
end

Any help appreciated .
Thanks,