as.character(list(NA))
As of R Under development (unstable) (2018-01-19 r74138): > as.character(list(NA)) [1] "NA" > is.na(as.character(list(NA))) [1] FALSE
As of R Under development (unstable) (2018-01-19 r74138): > as.character(list(NA)) [1] "NA" > is.na(as.character(list(NA))) [1] FALSE