UDA Multi-Row Attribute Group Error Message System Administrator must specify a combination of field

I've created a User Defined Attribute with two data items, a number and a date.  I want to track changes in the number with an effective date.  It has been defined as two columns, Multi-Row and both items have a value set, with the date being the Standard Date.

When I enter a single row, no issues.  When I attempt to enter the second row to indicate the value has changed and a date, I receive the following error:

"You cannot create more than one row of data for "xxx" until your system administrator specifies a combination of fields whose values uniquely identify a row of data for "xxx"."