JsonApiAdapter omitting null selection

We're using JsonApiAdapter on an ember-data project, and we're encountering that selecting the "prompt" (default, null value) for a <select> field results in the adapter omitting the null value for that property, when serializing the data.

The problem is, the presence of this null value is required to set that field to null in the backend.

Does anyone know how to "force" the presence of null values in the request body?