Fails with empty cluster

[SevereOverfl0w]

(#_rhizome.dot/graph->dot
  rhizome.viz/view-graph
  [:a]
  {}
  :node->descriptor (fn [n] {:label n})
  :cluster->descriptor (fn [n] {:label n})
  :node->cluster
  {:a :cluster-a}
  :cluster->parent
  {:cluster-a :cluster-b})

Results in an empty graph.

It's fixed by adding something into cluster-b:

(#_rhizome.dot/graph->dot
  rhizome.viz/view-graph
  [:a :b]
  {}
  :node->descriptor (fn [n] {:label n})
  :cluster->descriptor (fn [n] {:label n})
  :node->cluster
  {:a :cluster-a
   :b :cluster-b}
  :cluster->parent
  {:cluster-a :cluster-b})

I am trying to show a cluster of clusters.

Also removing the cluster parent from cluster-a of course fixes this (but without it being grouped.)