Add missing initializer

clientutil/backend.go

@@ -57,6 +57,7 @@ func NewBackend(config *BackendConfig) (Backend, error) {
 	return &shardedClient{
 		baseURL:   u,
 		tlsConfig: tlsConfig,
+		urls:      make(map[string]*url.URL),
 		shards:    make(map[string]*http.Client),
 	}, nil
 }