How to delete json key by JSONUpdater

Hi,

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How to delete the key "x" in a json attribute by JsonUpdater transformer:

{ "x" : "aa", "y" : "bb" }

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I send the feature both into the ports of "Document" and "Update", and set

Update Type: "Delete From Object"

JSON Path: json[*]

Object / Array Index: x

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but get warnings below:

2016-06-16 17:27:38| 0.2| 0.2|WARN |JSONUpdater(XQueryFactory2): The following error occurred while executing query number 1:

2016-06-16 17:27:38| 0.2| 0.0|WARN |JSONUpdater(XQueryFactory2): The following error occurred near line 3, column 21 of the query:

2016-06-16 17:27:38| 0.2| 0.0|WARN |JSONUpdater(XQueryFactory2): xs:string can not be promoted to parameter type json-item() of function json-item-accessor()

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Thanks