Property calculator: Calculate expression not working

Hello community,

I'm struggling with a property calculator rule.

I have a main object and 1 or more related documents with a property (float) "amount"

I would like to get the total amount in the main object (simple addition).

For that I created a rule with the expression: {RelatedObjType(Document).ForEach()}+{amount}

it should fill the property amount (same property so no conversion issue), but I get nothing in it when the rule is triggered.

I tried to initialize the main property to 0 but no difference.

When I test to send the result to Keywords property, I got the values I have in all my documents, so I don't understand where I'm wrong.

Cheers!