Help with changing column references in sumproduct

Hi,

I'm trying to fill down the following formula:
=$B2*SUMPRODUCT($B$2:$B$477;'COV matrix'!B$2:B$477)
While filling down, I want the formula to change from: $B2*SUMPRODUCT($B$2:$B$477;'COV matrix'!B$2:B$477) to $B3*SUMPRODUCT($B$2:$B$477;'COV matrix'!C$2:C$477) etc.
Now, it just stays the same.

Can somebody please help me.

Gilles