subsample.R question

Hi!

I would like to apply the subsample.R tool but I have some problems. I have import a .las file (read.las) but it appear to me next message:

Error in subsample(test_las_input) : 
  Wrong file is a  data.table , must be a characterWrong file is a  data.frame , must be a character

Please, could you help me with this?

Thanks in advance 