This results in a more friendly and useful error message than:
NoMethodError: undefined method `type_cast' for nil:NilClass
And is specific to the case where validate: false is passed as ActiveRecord would otherwise raise the error:
NoMethodError: undefined method `foo=' for #<Topic:0x007fd26e5238c8>
Raise a useful message when a value doesn't have a corresponding column.
Thanks @benpickles !