Membership Error on PayPal: "specify valid values for the A3, P3"

As admin, I have a monthly membership acct by default. When I try to upgrade the account to annual, after I click the PayPal payment button, I get the following error message on the PayPal site:

"Invalid Regular period. You must specify valid values for the A3, P3, and T3 parameters for a subscription."

What is causing this and how do I fix it?