Appsync resolver context not working, used literal strings somehow

Been using this resolver fine, but after an update at some point, the userId has been the literal string "${ctx.identity.cognitoIdentityId}" instead of being replaced with actual Id. Now I'm not sure how to make it work.

{
            "version" : "2017-02-28",
            "operation" : "GetItem",
            "key": {
              "userId": { "S": "${ctx.identity.cognitoIdentityId}" }
            }
}