data(Produc)
fm1 <-   diff(log(gsp))  ~ log(pcap) + log(pc) + log(emp)
fm2 <- I(diff(log(gsp))) ~ log(pcap) + log(pc) + log(emp)

plm(fm1, Produc) # runs
plm(fm2, Produc) # errors
## plm version 2.2-5
# Error in is.pbalanced.default(x) :
#   argument "y" is missing, with no default

## plm version 2.6-0
# Error in fwithin.default(x, g = effect, w = NULL, na.rm = na.rm) :
#   length(g) must match nrow(X)