Can I connect without a client-side certificate, and how?

I’m getting the following error:

x509_verify_cert() returned -9984 (-0x2700)

One of our engineers said “You’re not using a client-side certificate, are you?” Is it possible to make an https connection without using a certificate and, if so, how?

My version information is:

#define MBEDTLS_VERSION_NUMBER 0x020D0100
#define MBEDTLS_VERSION_STRING “2.13.1”
#define MBEDTLS_VERSION_STRING_FULL “mbed TLS 2.13.1”

RFC5246 days the client may send a zero-length certificate message, in lieu of a certificate but I don’t see to do this.