Can't eagerly load a polymorphic association defined in an STI parent class?

If I define an association (in my case a polymorphic one) in an STI
parent class, and try to eagerly load it in a subclass, I get the
following error:

ActiveRecord::ConfigurationError: Association name 'myassoc' was not
found; perhaps you misspelled it?

If I redefine the same association again in the STI subclass, it
works.

Is this a bug or what?

Probably.

If you post a pared-down set of model code and how you're trying to
load it; then we can see if we can help you spot it.