module 'ngx.upstream' not found in openresty 1.13.6

when I run the lua code like this:
`
stream{
    preread_by_lua_block{
        local upstream = require("ngx.upstream")
        ...
    }
}
` 
nginx throw the error: module 'ngx.upstream' not found

should I install the module upstream or is it a bug in openresty 1.13.3?