JSON payload for a Fault

We need to generate a JSON output in case of fault. RaiseFault policy is appropriate for the same. Following is an example policy
<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<RaiseFault async="false" continueOnError="false" enabled="true" name="invalid_login">
    <FaultResponse>
        <Set>
            <Payload contentType="application/json" variablePrefix="%" variableSuffix="#">
                {
                    "error": "Incorrect login credentials",
                    "source": "API error"
                }
            </Payload>
            <StatusCode>400</StatusCode>
        </Set>
    </FaultResponse>
</RaiseFault>

Why this so different  - please note the variablePrefix, variableSuffix being present even though the content type is application/json and no variables are used. contentType attribute does not have the same impact as in AssignMessage policy. The presence of {} leads to the interpretation of the data between them as variable name. The variablePrefix and variableSuffix change solves the problem.