R execution error : object of type 'externalptr' is not subsettable


I am executing the following R snippet through KNIME :

“R<-add.constraint(R, R$“ECV”, “<=”, 8000)”

and getting the following error:
Error in R$ECV : object of type ‘externalptr’ is not subsettable
Calls: add.constraint
Execution halted

Any idea about the same? If I execute it through the KNIME console itself, the same works fine.
The snippet that i use when directly executing from console is :
“lprec<-set.objfn(lprec, Optimization$“Revenue”)”

I am not sure, but is it possible that this function returns some kind of pointer or status that is assigned to R? Can you please try:
add.constraint(R, R$“ECV”, “<=”, 8000)
If this does not help, can you please provide a small sample dataset that demonstrates the problem.
Thanks, Thomas