I was following this tutorial to prepare data with domains / subtypes for use in Collector.
I therefore had an empty featureclass in my map. If I symbolised the featureclass (in this case by the Unique Values set up as one domain within the geodatabase) before sharing, the packaging would fail. If I left it as Single Symbol, it succeeded.
Is this expected? I can go on to create my map for Collector in AGOL using my desired symbology, so it's not a big problem, but I lost a lot of time tracking this down.