Postman API - /collections/id/responses endpoint 400 error

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This is the request body I am using:

{
    "name": "test_response",
    "responseCode": {
        "code": 200,
        "name": "OK"
    },
    "headers": [
		{
			"key": "content-type",
			"value": "application/json"
		}
	],
    "text": "{\n\"testKey\":\"testValue\"}\n",
    "language": "json",
	"requestObject": "{\"method\":\"POST\",\"url\":\"https://test_server.com/test/test_response\"}"
}