Aggregating is only supported in grids with an XPath source

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Dear everyone, I found an issue about aggregation,  I created a datagrid, and it has an integer type column named Amount. After I set the [Aggregate function] to Sum, the Error appeared. [CE0508  Aggregating is only supported in grids with an XPath source ] I use the Mendix Studio pro version 8.15.1. Why does the system prompt this error? who does have any suggestions? Thank you ！    
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Aggregates don't work for other data sources than database because they are computed using SQL queries.

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