ResourceSerializer serialized hash is nil

Just updated to rails 5.

In a controller test, using minitest:

#From factory girl
user = create(:user)

resource = UserResource.new(user), nil)

serializer = JSONAPI::ResourceSerializer.new(UserResource)

output = serializer.serialize_to_hash(resource)

The resource is:

p resource

#<UserResource:0x0055ba82bd2170 
@model=#<User id: nil, name: "Mitchell-Gerhold">, 
@context=nil, 
@reload_needed=false, 
@changing=false, 
@save_needed=false>

But the serialized output is:

p output

{
    :data => nil
}

Any ideas?

Start by reproducing the controller example and tracing JSONAPI::ResourceSerializer#serialize_to_hash with the shown UserResource state. Compare the resource's model and serializer input with the expected output, then verify that a regression check covers the nil :data result.