Overstatement of Purchases

Hurley's accountant recorded a purchase of merchandise for $9,000 in 2020 that applied to 2021. The physical inventory for 2020 was correctly stated. The company uses the periodic inventory method. The entry on December 31, 2021, to correct for this error, assuming that Harley has not closed the books for 2021, is:

Purchases	9,000
Retained Earnings		9,000

Question: Why is the purchase account debited for an overstatement of purchases? I thought the normal balance

for this account was a debit.