Usually, the reason behind such error is that you can sum only Numeric types. Please, review the following article in order to get more information on the possible solution - Exception with Sum AggregateFunction
If the aforementioned article does not resolve the issue at your end would it be possible to provide more details on your exact setup. Sending a sample project would be the best approach as we would be able to have a more detailed look and further investigate the issue.
I am looking forward to your reply, Karthikeyan.
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