GetExternalContactAction returns empty map when calling internal lookup endpoint

Trying to use the GetExternalContactAction in Architect to fetch customer details before routing. The flow hits the action, calls our internal API, but the contact map stays empty. Here’s the config:

{
 "method": "GET",
 "url": "https://api.internal/customers/lookup?phone=${dn}"
}

The endpoint works fine in Postman and returns valid JSON. No error in the logs, just null data. What am I missing?