MongoDB summing an array

I have a timeseries documents in a mongodb collection. One of the fields is an array containing 10 values. I want to plot the sum of these 10 values against date.
 
Using BIRT under Eclipse (4.6.0) I created a computed column as follows:
 
row["return"].reduce(function(a, b) {return a + b;}, 0)
 
validation of this calculation works but when I include the computed value in a chart a BIRT exception is generated saying that the reduce method cant be found.
 
Does anyone have any ideas about what I'm doing wrong or how I could achieve what I want using another approach?

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Comments

Clement Wong

What data type is "return" defined in the Data Set's Output Column?
 
Next, when you go to your Data Set's Edit Data Set dialog, what output do you see when you "Preview Results"?
 
I'm guessing that row["return"] isn't an true array and that you'll see an issue in the "Preview Results" too.