Trying to update TestCase using TCMS API

I am having issues updating test cases. This is my python code:

for test_case in rpc_client.exec.TestCase.filter({'pk': x}):
    rpc_client.exec.TestCase.update(x, test_case)

I am getting this error:

 <Fault -32603: 'Internal error: Product ID is required for category'>

Can someone explain to me what is the issue? For testing purposes I just get the text case and then I call the update function. I am not sure what Product ID I have to include. Official documentation does not have any information about what this attribute actually stands for.

Thank in advance.