AVERAGEX Conditional Logic

Hi all,

I'm having trouble getting an averagex function to work correctly, my data has one row per person, I need a function that checks a flag column then choses which column to sum as appropriate.

At the moment it seems to be evaluating for more than one person at a time as I'm getting an error about multiple values. Code is below, can anyone sugest a solution?

=AVERAGEX (
        VALUES ( table[Person_ID] ),
        IF (
            values(table[flag]) = 1,
            sum( table[Col1] ),
            sum(table[Col2])
        )
    )

Than you