Why can't i use ngx.socket.tcp() in init_by_lua phase?

I need to read api definition from unix socket(postgresql) and set it as a variable in shared dict. Is there a reason why this is forbidden?

    local pg = pgmoon.new({
        socket_type = "nginx",
        host = table.concat({"unix:", config.socket}),
        database = config.database,
        user = config.user
    })

    assert(pg:connect())
    local res = assert(pg:query("select info from testy limit 1"))
    return res

crashes with

nginx: [error] init_by_lua_file error: /usr/local/share/lua/5.1/pgmoon/socket.lua:99: no request found
stack traceback:
        [C]: in function 'tcp'
        /usr/local/share/lua/5.1/pgmoon/socket.lua:99: in function 'new'
        /usr/local/share/lua/5.1/pgmoon/init.lua:1093: in function '__init'
        /usr/local/share/lua/5.1/pgmoon/init.lua:1101: in function 'new'
        ./lua/store.lua:7: in function 'read'
        .../peter/Documents/resty-engine/src/lua/lifecycle/init.lua:8: in function 'reloadAPI'
        .../peter/Documents/resty-engine/src/lua/lifecycle/init.lua:16: in main chunk

same problem will occur if you just insert

local socket = ngx.socket.tcp()

into init_by_lua_block or file

According to the documentation,

> The cosockets are currently [...] disabled in the init_by_lua* and init_worker_by_lua* directive contexts