Paypal Express fails if no "additional contribution" entered on a membership signup page

Using PaypalPro as a payment processor and a contribution page with membership block and contribution amounts section enabled with "seperate memberrship payment" checked.

If a contributor chooses to pay via paypal express (clicking the paypal button graphic) they are given this error:

Payment Processor Error message
: 10400:Transaction refused because of an invalid argument. See
additional error messages for details. Order total is missing.

NOTE: this only is an error when "Separate membership payment" is checked because nothing is required to be entered in the "additional contributions" box. if "seperate membership payment is not checked then the contribution amount must be entered and there is no error given.