When a FutureValue has to call its ExecuteOnEval delegate, it fails with an unable to convert exception from a scalar/entity to a IQueryable of the scalar/entity.
Current 5.0.0 code base seems to affect ExecuteOnEval only when Value is obtained from First/FirstOrDefault/Single/SingleOrDefault, like in
session.Query<DomainClass>().Select(e => e.Id).ToFutureValue(q => q.FirstOrDefault()).Value
This is done by ProcessFirstOrSingleBase from ResultOperatorProcessors namespace. It defines a PostExecuteTransformer on the IntermediateHqlTree taking an IQueryable as input and yielding the scalar/entity as output. This transformer ends up affected to ExecuteOnEval.
But current implementation of FutureValue call ExecuteOnEval on a scalar/entity value instead of an IQueryable.
This trouble was discovered while attempting to solve NH-3850. (The solution I am working on for this other bug involves adding PostExecuteTransformer for scalar queries like Count/Min/Max/Sum, which has caused some FutureValue tests to start failing due to this bug with FutureValue and PostExecuteTransformer.)