Can't get app:include working when writing in lua

[FourierTransformer]

I was trying to replicate the app:include from the lua workshop video, but couldn't get it working...

local lapis = require("lapis")
local app = lapis.Application()

-- add my api
app:include("api.v1")

return app

and when I try running it, I get a lua entry thread aborted: runtime error: ./app.lua:5: attempt to call method 'include' (a nil value)

Am I doing something wrong or is there a bug?