ExcelBanter - Error when using the SUMXMY2 function

This is what I am trying to do:

I have two columns on an Excel spreadsheet: Column A represent Years (and
are listed 1,2,3,4...etc). Column B lists Sales per year in thousands
(301,343,289,...etc).

I created a Column C for the two month averages of Column B. I am pretty
sure that I have done this correctly since all the values are correct and
when I double-check the formula in each cell the formula is correct.

The problem is when I try to get an sbe for Column C. This is the formula I
am using:

=SUMXMY2(B5:B22,D5:D22)/COUNT(D5:D22)

B5:B22 is Column B (Original reports for sales per year)
D5:D22 is Column C (Two-month averages)

The value I get for the equation is wrong - it is a value that is almost
double the highest sale value in either Column B or Column C.

The only thing that seems like a red-flag in my calculations is the warning
(exclamation point within a yellow triangle) that states "the formula in
the cell refers to a range that has additional numbers adjacent to it". I
have tried to ignore the error, etc but I get the same answer no matter what
I try.

I remain an Excel beginner (this exercise is part of a class on
forecasting) so I welcome any help - and the clearer the explanation, the
better it will be for me!

Thanks for all help offered-