Error in `setDT` from `data.table` package

Question

It seems like setDT returns an error whenever you are trying to run it on a stored data in the datasets package. For example

library(data.table)
setDT(CO2)

## Error in assign(name, x, parent.frame(), inherits = TRUE) : 
##   cannot change value of locked binding for 'CO2'

The strangiest thing is that if you rerun setDT(CO2) it will work

So I've looked in the source code of setDT and tried to reproduce the error

x <- CO2
name <- as.character(substitute(x))
assign(name, x, parent.frame(), inherits = TRUE)

Which worked and didn't return any error. My guess is that the parent.frame() is the one that causing it, but I can't figure out by myself whats going on back stage. Also, I can't understand why setDT(CO2) doesn't return an error on the second run.

My sessionInfo()

## R version 3.0.3 (2014-03-06)
## Platform: x86_64-w64-mingw32/x64 (64-bit)
## 
## locale:
##   [1] LC_COLLATE=English_United States.1252  LC_CTYPE=English_United States.1252    LC_MONETARY=English_United States.1252
## [4] LC_NUMERIC=C                           LC_TIME=English_United States.1252    
## 
## attached base packages:
##   [1] stats     graphics  grDevices utils     datasets  methods   base     
## 
## other attached packages:
##   [1] data.table_1.9.2
## 
## loaded via a namespace (and not attached):
##   [1] plyr_1.8       reshape2_1.2.2 stringr_0.6.2  tools_3.0.3   

Answer 1

With commit 1320, setDT now returns a friendly error that the object can not be modified by reference when it's binding is locked. From NEWS, No:37 under bug fixes for 1.9.3:

setDT now provides a friendly error when attempted to change a variable to data.table by reference whose binding is locked (usually when the variable is within a package, ex: CO2). Closes #475. Thanks to David Arenburg for filing the report here on SO.