How to enforce the default values of a nested dict?

[db0]

I have the following model definition

        self.root_model_generation_payload_stable = api.model('ModelPayloadRootStable', {
            'sampler_name': fields.String(required=False, default='k_euler_a',enum=["k_lms", "k_heun", "k_euler", "k_euler_a", "k_dpm_2", "k_dpm_2_a", "k_dpm_fast", "k_dpm_adaptive", "k_dpmpp_2s_a", "k_dpmpp_2m", "dpmsolver"]), 
            'toggles': fields.List(fields.Integer,required=False, example=[1,4], description="Obsolete Toggles used in the SD Webui. To be removed. Do not modify unless you know what you're doing."), 
            'cfg_scale': fields.Float(required=False,default=5.0, min=-40, max=30, multiple=0.5), 
            'denoising_strength': fields.Float(required=False,example=0.75, min=0, max=1.0, multiple=0.01), 
            'seed': fields.String(required=False,description="The seed to use to generete this request"),
            'height': fields.Integer(required=False, default=512, description="The height of the image to generate", min=64, max=3072, multiple=64), 
            'width': fields.Integer(required=False, default=512, description="The width of the image to generate", min=64, max=3072, multiple=64), 
            'seed_variation': fields.Integer(required=False, example=1, min = 1, max=1000, description="If passed with multiple n, the provided seed will be incremented every time by this value"),
            'post_processing': fields.List(fields.String(description="The list of post-processors to apply to the image, in the order to be applied",enum=["GFPGAN", "RealESRGAN_x4plus"]),unique=True),
            'karras': fields.Boolean(default=False,description="Set to True to enable karras noise scheduling tweaks"),
        })
        self.input_model_generation_payload = api.inherit('ModelGenerationInputStable', self.root_model_generation_payload_stable, {
            'steps': fields.Integer(default=30, required=False, min = 1, max=500), 
            'n': fields.Integer(default=1, required=False, description="The amount of images to generate", min = 1, max=20), 
        })
        self.input_model_request_generation = api.model('GenerationInput', {
            'prompt': fields.String(required=True,description="The prompt which will be sent to Stable Diffusion to generate an image", min_length = 1),
            'params': fields.Nested(self.input_model_generation_payload, skip_none=True),
        })

I have also set validate=True in my expect()

I expected the value of the nested field params to be filled with its default values, when either it doesn't exist, or it exists but it does not contain said default values.

However that field is just ignored. If it's passed without some of the values needing defaults, they are ignored. If it's not passed, then no dict is added either.

How can enforce the behaviour that the params dictionary will always exist with at least its default values?