Mailchimp Invalid Resource

I'm getting this error when trying to add a new subscriber to Mailchimp list. What am I doing wrong?

{"type":"http://developer.mailchimp.com/documentation/mailchimp/guides/error-glossary/","title":"Invalid Resource","status":400,"detail":"The resource submitted could not be validated. For field-specific details, see the 'errors' array.","instance":"","errors":[{"field":"","message":"Schema describes object, NULL found instead"}]}

<cfhttp url="https://us9.api.mailchimp.com/3.0/lists/{list_id}/members" method="POST" username="xxxxxx" password="xxxxxxxxxxxxxxx-us9">

            <cfhttpparam name="output" value="json" type="url">

            <cfhttpparam name="method" value="listSubscribe" type="URL">

            <cfhttpparam name="email_address" value="test@mail.com" type="url">

            <cfhttpparam name="status" value="pending" type="url">

            <cfhttpparam name="email_type" value="html" type="url">

            <cfhttpparam name="double_optin" value="FALSE" type="url">

            <cfhttpparam name="merge_vars[fname]" value="Test" type="url">

            <cfhttpparam name="merge_vars[lname]" value="NewSub" type="url">

</cfhttp>