Cannot get any assignments from waapi

Question

by running ak.wwise.core.switchContainer.getAssignments

I didn't get any response. here's my code:

        results = self._client.call("ak.wwise.core.switchContainer.getAssignments", {"id": "{97F554AF-3252-4FF1-B0C3-F61F73BCD240}"})

I confirm the guid is attached to a music switch container. but I got nothing from this api.