Why can't I modify host_collections via API call in Satellite 6.3?

Issue

Attempting to use curl to add a content host to a host collection results in a message similar to:

{
    "displayMessage": "undefined method `map' for \"4\":String\nDid you mean?  tap",
    "errors": [
        "undefined method `map' for \"4\":String\nDid you mean?  tap"
    ]
}