Membership Price Set - Text/Quantity field does not function

When a Text/Quantity field is included in Membership Price set, users that select a membership and then enter any value into that field are shown the error message:

"You have selected multiple memberships for the same organization or entity. Please review your selections and choose only one membership per entity."

What I did -
Created a membership price set with two price fields; (1) Membership Levels, with radio buttons for selecting the membership level, and (2) Additional categories, a text/numeric quantity field for the additional number category listings desired.

Expected outcome-
Membership be created according to the radio field selected; value for the text/quantity field is included in total cost.

Actual outcome-
Selecting only the radio button for the membership - membership recorded, changes saved.
Selecting the radio button for the membership AND entering a number the text/quantity field - error message about multiple memberships.
Not selecting a radio button AND entering a number in the text/quantity field - error message "Please select at least one membership option"

This affects versions 3.4.8 and 4.0.8 (able to replicate on http://d7.demo.civicrm.org/). Version 3.4.7 achieves the expected outcome without any error messages.

Also see forum post at: http://forum.civicrm.org/index.php/topic,23188.msg97484.html#msg97484