Assign Message - AssignVariable JSON content

[VAP]

Hi,

I want to save the request body sent to the target server, for latter log. For that I used a AM policy in the postflow of the target 

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<AssignMessage async="false" continueOnError="false" enabled="true" name="AM-SetVarsToTarget">
    <IgnoreUnresolvedVariables>true</IgnoreUnresolvedVariables>
    <DisplayName>AM-SetVarsToTarget</DisplayName>
    <AssignVariable>
        <Name>target.request.body</Name>
         <Value>DefaultReq</Value>
        <Ref>message.content</Ref>
    </AssignVariable>
    <AssignVariable>
        <Name>target.request.querystring</Name>
        <Ref>message.querystring</Ref>
    </AssignVariable>
</AssignMessage>

But when I try to use the target.request.body variable in the PostClientFlow, for logging purposes the variable is empty. When using the debug view I see a unequal sign next to the variable

Any clues of what I'm doing wrong here?

Thanks!