Authorize returns error on membership additional contribution, but processes card anyway

Using Authorize.net, following contribution page settings:

1. Contribution Amounts section enabled

2. Allow other amounts

3. Require Membership Signup

4. Separate Membership Payment

Combine to generate the following error upon submission to Authorize.net "require a valid amount" when a membership is selected but no 'additional contribution' is entered.

Turning off option #1, #2 and #4 allows a successful transaction.

Because of the error, it appears to the transaction was failed, but in fact the transaction was a success and the credit card was charged. This resulted in several repeated transactions and great confusion in our instance.