diff --git a/clientapi/routing/register.go b/clientapi/routing/register.go
index 5235e9092..695425029 100644
--- a/clientapi/routing/register.go
+++ b/clientapi/routing/register.go
@@ -513,13 +513,6 @@ func Register(
 		return handleGuestRegistration(req, r, cfg, userAPI)
 	}
 
-	// Don't allow numeric usernames less than MAX_INT64.
-	if _, err = strconv.ParseInt(r.Username, 10, 64); err == nil {
-		return util.JSONResponse{
-			Code: http.StatusBadRequest,
-			JSON: spec.InvalidUsername("Numeric user IDs are reserved"),
-		}
-	}
 	// Auto generate a numeric username if r.Username is empty
 	if r.Username == "" {
 		nreq := &userapi.QueryNumericLocalpartRequest{
diff --git a/sytest-whitelist b/sytest-whitelist
index 492c756ba..9cdcbe8a6 100644
--- a/sytest-whitelist
+++ b/sytest-whitelist
@@ -709,6 +709,7 @@ PUT /rooms/:room_id/redact/:event_id/:txn_id is idempotent
 Unnamed room comes with a name summary
 Named room comes with just joined member count summary
 Room summary only has 5 heroes
+registration is idempotent, without username specified
 registration is idempotent, with username specified
 Setting state twice is idempotent
 Joining room twice is idempotent