Post request to NetworkInfrastructures not working

Issue in API Playground
I'm attempting to add a dedicated IP address for a customer using the following, but when I click on submit, nothing happens:

But I'm expecting to see something similar to this when I submit:

The console shows this error:

However, I don't think that this is invalid syntax as I've used this exact syntax before. I'm not sure what is wrong with it... it's not very fancy.
Subsequently, if I click on the "Get Request" button for networkInfrastructures, nothing happens. Nothing is logged to the console or network in the debugger.