Cannot add a NOT NULL column with default value NULL

I'm testing a laravel site using composer and I've followed the following tute:

http://ift.tt/2agfeOw

I'm getting the following error:

[Illuminate\Database\QueryException] SQLSTATE[HY000]: General error: 1 Cannot add a NOT NULL column with default value NULL (SQL: alter table "markers" add column "user_id" integer not null)

Thanks in advance.