SystemError: null argument to internal routine

I am trying to figure out the conditions under which I get that
SystemError. Here is a simple way that I found to reproduce the
problem:

>>> import psycopg2
>>> psycopg2.__version__
'2.3.2 (dt dec pq3 ext)'
>>> conn = psycopg2.connect("...")
>>> cur = conn.cursor()
>>> cur.execute(";")
Traceback (most recent call last):
File "<stdin>", line 1, in ?
SystemError: null argument to internal routine

I am getting the same exception during a call to copy_expert. How would
the data I pass in there trigger the same exception as when I pass a
single semicolon? I am guessing that it does not even make it over the
wire to the postgres server when the exception is raised. Thanks.

-eric

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