Client Side 'sum' not functioning as expected

This may be a bug but I wanted to see if someone else could verify it for me or tell me if this is my problem. I updated to v2.5.2 a few days ago and shortly afterwards I have begun to have a big problem on my site.

I use a function to sum the fields of a data set returned by a server connect, like this:
sc_list_params.data.params.sum(‘amount’)

This has stopped working and I am getting a referenceError in the console.

These both return a value:
sc_list_params.data.params[0].amount
sc_list_params.data.params[1].amount

The error I am seeing is:

    parser.js:384 ReferenceError: item is not defined
    at array.js:242
    at Array.reduce (<anonymous>)
    at n.sum (array.js:240)
    at parser.js:708
    at parser.js:449
    at Object.dmx.parse (parser.js:382)
    at Object.dmx.parse (parser.js:369)
    at n.<anonymous> (BaseComponent.js:316)
    at Array.forEach (<anonymous>)
    at n.$updateBindings (BaseComponent.js:314)

To reproduce, create a server connect that retuns a number of rows that include a numeric field. Sum the field and see what happens.