Rails 4.2: NoMethodError: undefined method `type' for "numeric":String

Trying to upgrade to Rails 4.2 I got this error all over my tests.
google failed me, but after many hours of puzzling, I realised I had
decimal fields in two models with no precision and scale set. In Rails
4.1, this was not a problem, but Rails 4.2 was not happy with that.

Thought I would post it here in case anyone else runs into it.