QUIC is a new transport protocol over UDP which is recently became an IETF RFC. Our security analysis of the Connection ID mechanism in QUIC reveals that the protocol is underspecified. This allows an attacker to count the number of server instances behind a middlebox, e.g., a load balancer. We found 4/15 (~25%) implementations vulnerable to our enumeration attack. We then concretely describe how an attacker can count the number of instances behind a load balancer that either uses Round Robin or Hashing.