The Mapper component does not currently deserialize IList properties. It throws this error:
This can be seen in master in test Cassandra.IntegrationTests.Mapping.Tests.Fetch.Fetch_UsingCqlObject() by changing Author.Followers from a List<string> to an IList<string>.
This change would fix it, but I haven't really though through all the knock-on effects so won't submit a PR just yet.
Thanks for the repro information and pointing to the source of the issue.
merged to oss
merged to dse