pl/python, plpy and nrows()

Hallo,

I do
res = plpy.execute("select * from koordinaten")

print("res: "+repr(res))

print("res.nrows(): "+repr(res.nrows()))

and on the stdout of the postmaster-process I get:
res: <PLyResult object at 0x8767350>
ERROR: plpython: Call of function `__plpython_procedure_sms_46221'
failed.
exceptions.SystemError: error return without exception set

It seems, that it just stops execution, when I call nrows().
What can I do about it?

Ciao,
Gerhard