Traceback (most recent call last):
  File "render_barcode_datamatrix.py", line 652, in <module>
    e.affect()
  File "/usr/share/inkscape/extensions/inkex.py", line 215, in affect
    self.effect()
  File "render_barcode_datamatrix.py", line 647, in effect
    encoded = encode( so.TEXT, (so.ROWS, so.COLS) ) #get the pattern of squares
  File "render_barcode_datamatrix.py", line 68, in encode
    data_nrow, data_ncol, reg_row, reg_col, nd, nc, inter = get_parameters( nrow, ncol )
TypeError: 'NoneType' object is not iterable