API key not valid. Please pass a valid API key

curl 'https://www.googleapis.com/youtube/v3/subscriptions?key=$KEY'
{
  "error": {
    "code": 400,
    "message": "API key not valid. Please pass a valid API key.",
    "errors": [
      {
        "message": "API key not valid. Please pass a valid API key.",
        "domain": "global",
        "reason": "badRequest"
      }
    ],
    "status": "INVALID_ARGUMENT",
    "details": [
      {
        "@type": "type.googleapis.com/google.rpc.ErrorInfo",
        "reason": "API_KEY_INVALID",...