I tested it with a million entries and it worked fine.
Here are the lines I added to the primes table in Objects/mappingobject.c:
65521, 131071, 262139, 524287, 1048573, 2097143,
4194301, 8388593, 16777213, 33554393, 67108859,
134217689, 268435399, 536870909, 1073741789,
Thanks!
-steve