FluentMigrator: Altering column to be nullable doesn't work

I have a column that was earlier declared as NOT NULL in SQLite database. I added a simple migration in FluentMigrator:

Alter.Table("table")
    .AlterColumn("column_time").AsDateTime().Nullable()
    ;

Yet, in produces "ALTER TABLE" query without NULL directive, so column doesn't become nullable.

Is it an issue in FluentMigrator or I am doing something wrong?