Connecting to Ping Federate with Certification authentication

[#1]

I have service callout to Ping Federate to validate an access token. I have this successfully working with basic authentication (client ID and password). but now I would like to use TLS authentication.

My client ID on Ping is set up with a trusted cert.

I have imported this cert into Apigee

Below is my callout

<HTTPTargetConnection>
        <Properties/>
        <URL>https://PING_FEDERATE:9031/as/introspect.oauth2</URL>
        <SSLInfo>
            <Enabled>true</Enabled>
            <TrustStore>TRUST_STORE_NAME</TrustStore>
        </SSLInfo>
    </HTTPTargetConnection>

But with these settings I get an error message of "PING_CLIENT_ID is configured to use client certificates but there were no client certificates sent in the request"

Any pointers where i'm going wrong would be much appriacted