Fields cannot be freely named

[mvcds]

When I try to create an entity like this...

export default class Foo extends Entity {
  static SCHEMA = {
    id: PropTypes.string,
    expression: {
      validator: PropTypes.string,
      defaultValue: ''
    }
  };
};

...the instantiated object contains the field expresion, but it is an object with two properties: expression (empty string) and errors (false).

Renaming the field to anything else caused it to behave as expected.

The same happens if I name the field as value. It gets two properties: expression (empty string) and errors (false).