Data Refresh Error - We cannot convert the value "[Function]" to type Function

I have a column in my dataset for timestamp which comes in as an epoch datetime. I have another column that is the duration (in milliseconds).

I've also created a new column that adds the duration to the timestamp (to get the end date). The formula is:

[timestamp] + #duration(0,0,0,([duration]/1000))

This works perfectly in PowerBi Desktop but when I publish to the service, I get the following error when refreshing the dataset:

We cannot convert the value "[Function]" to type Function. Table: Calls.

Any ideas on why this is? My other Custom Columns and functions all work fine - it's just this one.