Custom Policy - Response headers not captured from the API's Response message

I am trying to develop a custom policy for my client. Everything works fine, but when I try to capture headers list from the response message, it always comes as empty - Headers:[]. The logger statement below prints the complete "message" structure, but headers always empty.

<?xml version="1.0" encoding="UTF-8"?>

<mule xmlns:doc="http://www.mulesoft.org/schema/mule/documentation"

xmlns:ee="http://www.mulesoft.org/schema/mule/ee/core"

xmlns:http="http://www.mulesoft.org/schema/mule/http"

xmlns="http://www.mulesoft.org/schema/mule/core"

xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"

xmlns:http-policy="http://www.mulesoft.org/schema/mule/http-policy"

xsi:schemaLocation="http://www.mulesoft.org/schema/mule/core http://www.mulesoft.org/schema/mule/core/current/mule.xsd

http://www.mulesoft.org/schema/mule/http-policy http://www.mulesoft.org/schema/mule/http-policy/current/mule-http-policy.xsd

http://www.mulesoft.org/schema/mule/http http://www.mulesoft.org/schema/mule/http/current/mule-http.xsd"> 

<http-policy:proxy name="{{{policyId}}}-custom-policy">

<http-policy:source>

<http-policy:execute-next/>

<logger level="INFO" message="#message" />

</http-policy:source>

</http-policy:proxy></mule>

The output in logs:

{ payload=org.mule.runtime.core.internal.streaming.bytes.ManagedCursorStreamProvider@4180b81f

mediaType=application/json; charset=UTF-8

attributes=org.mule.extension.http.api.HttpResponseAttributes

{

Status Code=200

Reason Phrase=null

Headers=[]

}

attributesMediaType=*/*

}

The API does return Headers as expected, when debugged in Anypoint Studio.

Any idea?

Thanks!