"Cannot convert" error when printing

[blookot]

Hi,
I'm running last version and whenever I try to print I'm getting "Cannot convert" errors for a simple file either plain .txt or translated to .pcl format (whatever language PS, PCL or PJL I use to launch pret.py).
Here is the error:

192.168.10.200:/> print toto.pcl
Converting 'toto.pcl' to pcl format
Cannot convert (convert: no images defined `pcl:-' @ error/deprecate.c/ConvertImageCommand/3366.)
Traceback (most recent call last):
  File "/home/kali/PRET/printer.py", line 138, in onecmd
    cmd.Cmd.onecmd(self, line)
  File "/usr/lib/python2.7/cmd.py", line 221, in onecmd
    return func(arg)
  File "/home/kali/PRET/printer.py", line 817, in do_print
    data = c.UEL.encode() + data + c.UEL.encode()
TypeError: cannot concatenate 'str' and 'NoneType' objects
None
Program Error (cannot concatenate 'str' and 'NoneType' objects)

Note when using the same file in .prn format I don't get the answer, I only get the following output (and the printer isn't printing):

192.168.10.200:/> print toto.prn
Converting 'toto.prn' to pcl format
H

Thanks for your help!