Using data.table package in R to sum over columns - getting GForce sum(gsum) error

here is a data.table:

Date     colA  colB  colC  .... month    year
01/23/15  2323  2323 2323        january  2015
.......

On this data.table Im trying to: 1) Sum all column values by month and then year 2) In the subset returned I want to exclude the Date column

I have set keys on the DT as follows:

setkey(DT, month, year)

Now Im running this command to achieve the operations listed in steps 1 & 2 above:

DT[ ,lapply(.SD, sum, na.rm=TRUE), by=.(month , year), .SDcols= 2:(length(colnames(DT))-2) ]

I got the above example from this SO post here.

When I run this..... I get the following error:

Error in gsum(`colA`, na.rm = TRUE) : 
  Type 'character' not supported by GForce sum (gsum). Either add the prefix base::sum(.) or turn off GForce optimization using options(datatable.optimize=1)

Im not sure what this means and how to debug it.......

Any assistance would be appreciated. Thanks

via Chebli Mohamed