FastR crashes with message `unimplemented Rf_asS4` when using package `rkdb`

[komsit37]

I am trying FastR with package https://github.com/KxSystems/rkdb. This is a follow up from an installation issue in #117. The package works in most cases, but I ran into an issue where FastR crashes when return datatype is nanotime.

> library(rkdb)
> h = open_connection('localhost',5555)
> execute(h, '2019.01.01')
[1] "2019-01-01"
> execute(h, '2019.01.01D00:00')
unimplemented Rf_asS4