[mbed-tls] Connect using IP address instead of URL "509 - Certificate verification failed, e.g. CRL, CA or signature check failed"

I am using MbedTLS client code based on this:

https://github.com/machinezone/IXWebSocket/blob/master/ixwebsocket/IXSocketM...

I am connecting to a server via it's URL. However, I would like to connect directly using an IP address returned from running the traceroute command on the URL.

So I replaced the URL with the IP address. However, MBedTLS fails on the handshake:

https://github.com/machinezone/IXWebSocket/blob/master/ixwebsocket/IXSocketM...

I get the error:

"error in handshake : X509 - Certificate verification failed, e.g. CRL, CA or signature check failed"

If I revert back to URL, it works. The IP address does exist.

How can I connect using the IP address, instead of the URL?