associationtypemismatch expected got array

I'm building an app a bit at a time. Sufficient time elapses between development effort that I forget some basic stuff.

Something that I haven't forgotten is how often I come across this error when I've made some changes to my models and edit forms:

ActiveRecord::AssociationTypeMismatch (MyModelName(#-613518908) expected, got Array(#-608606448)):

This typically means that I've forgotten:

accepts_nested_attributes_for

in my model.