ffdf subset works in RGui but not RStudio

ffbase subset() works in RGui R version 3.6.0 (2019.04.26), but not RStudio Version 1.2.1335, Windows 10,

library(ff)
library(ffbase)

create an ff data frame from iris data

iris.ffdf <- as.ffdf(iris)

subset the ff data frame, want only setosa

subset(iris.ffdf, Species=="setosa")

works fine in base RGui(64-bit), returns 50 row data frame

but in RStudio get this error

Error in if (by < 1) stop("'by' must be > 0") :
missing value where TRUE/FALSE needed
In addition: Warning message:
In chunk.default(from = 1L, to = 150L, by = 5124095576030431, maxindex = 150L) :
NAs introduced by coercion to integer range