[datatable-help] `:=` Fails When Called From Function Inside a Package

[Michael Smith]

All,

This is my first post on this list, thanks to all those who have made
data.table such a wonderful package.

I have a simple function that looks like this (for reproducibility):

myfun <- function() {
  DT <- data.table(a = 1:4, b = 5:8)
  DT[, x := a + 2]
  DT
}

It works perfectly fine, until I add it to a custom R package that I
just wrote. If I add it to the package and try to call it, I get the
following error:

> mypackage::myfun()
Error in `:=`(x, a + 2) :
  Check that is.data.table(DT) == TRUE. Otherwise, := and `:=`(...) are
defined for use in j, once only and in particular ways. See help(":=").

Not really sure what's going on here. Any ideas? Is this a bug or am I
doing something wrong?

Thanks,

M