PyGreSQL NULLs for integer field

Question

I am trying to insert some rows into a table which has a field of integers, which can be NULL:

cur.execute("INSERT INTO mytable (id, priority) VALUES (%(id)d, %(priority)d)", \
            {'id': id, 'priority': priority})

The priority variable is either an integer or None. This works when priority has an integer value, however, when it is None I get the following error:

internal error in 'BEGIN': int argument required

I notice that for string formatting for Python in general you cannot use None as the value of an integer to be formatted - it throws the same error that pgdb is throwing. However, when I change the formatting string to %(priority)s the error changes to:

internal error in 'BEGIN': unsupported format character 'W' (0x57) at index 60

I guess that's because I'm then trying to import a string into an integer field.

How do I import the NULL values?