Ember Data "modelFor" error when serializing models

I am using ember 1.13, ember-cli and ember data 1.19.1 and I am experiencing an error that is difficult to track down due to being unspecific. The error occurs when I do the following:

this.get('model').forEach(function(mymodel) {
    applications.push(mymodel.serialize({includeId: true}));
});

and a snippet of the error itself is:

Uncaught TypeError: Cannot read property 'modelFor' of
undefinedember$data$lib$system$model$$default.reopenClass.typeForRelationship @
ext.js:171ember$data$lib$system$model$$default.reopenClass._findInverseFor @     
ext.js:214ember$data$lib$system$model$$default.reopenClass.inverseFor @ 
ext.js:205Ember.Mixin.create.removeEmbeddedForeignKey @ embedded-records-mixin.js:317
Ember.Mixin.create.serializeBelongsTo @ embedded-records-mixin.js:199

I have managed to discover that the model this error is occurring on ("mymodel") only throws the error when relationships to another model are included, e.g.

quickQuote: DS.belongsTo('iso-application-status')

I assume this message relates to a misconfiguration of one of my models/serializers but I've been through and can't find anything amiss. The "mymodel" model is called "my-model" and looks like this:

import DS from 'ember-data';

export default DS.Model.extend({
    statusId: DS.attr('number'),
    status: DS.belongsTo('iso-application-status')
});

and the serializer for this is also called "my-model" and looks like this:

import DS from "ember-data";

export default DS.ActiveModelSerializer.extend(DS.EmbeddedRecordsMixin, {
    attrs: {        
        status: {embedded: 'always'}
    }
};

The related model is "iso-application-status" and looks like this:

import DS from "ember-data";

export default DS.ActiveModelSerializer.extend(DS.EmbeddedRecordsMixin, {
    attrs: {
        myModels: {async: true},
    }
});

Does anyone know what I have done wrong please?

Thanks.