diff --git a/api/server/router/swarm/cluster_routes.go b/api/server/router/swarm/cluster_routes.go
index a70248860208f..5d460741f1b87 100644
--- a/api/server/router/swarm/cluster_routes.go
+++ b/api/server/router/swarm/cluster_routes.go
@@ -23,6 +23,12 @@ func (sr *swarmRouter) initCluster(ctx context.Context, w http.ResponseWriter, r
 	if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
 		return err
 	}
+	version := httputils.VersionFromContext(ctx)
+	// DefaultAddrPool and SubnetSize were added in API 1.39. Ignore on older API versions.
+	if versions.LessThan(version, "1.39") {
+		req.DefaultAddrPool = nil
+		req.SubnetSize = 0
+	}
 	nodeID, err := sr.backend.Init(req)
 	if err != nil {
 		logrus.Errorf("Error initializing swarm: %v", err)