UnboundID LDAP SDK startTLS "No extended operation handler" error

Question

Created an in-memory UnboundID LDAP server. The server listens on an unencrypted port and a TLS port. I can bind to it with LDAP on the first port, and LDAPS on the second port. However, I cannot connect to it using startTLS. The error is:

LDAPException(resultCode=53 (unwilling to perform), errorMessage='No extended operation handler is defined for extended request OID '1.3.6.1.4.1.1466.20037'.', diagnosticMessage='No extended operation handler is defined for extended request OID '1.3.6.1.4.1.1466.20037'.')

Here is how the server is configured:

 try {
            InMemoryListenerConfig ldapsConfig = InMemoryListenerConfig.createLDAPSConfig("ldaps",
                    LDAPS_PORT,
                    getServerSSLContext().getServerSocketFactory());
            InMemoryListenerConfig ldapConfig = InMemoryListenerConfig.createLDAPConfig("ldap",
                    LDAP_PORT);
            InMemoryDirectoryServerConfig serverConfig serverConfig = new InMemoryDirectoryServerConfig(BASE_DN);
            serverConfig.addAdditionalBindCredentials(BIND_RDN, BIND_PASSWORD);
            serverConfig.setListenerConfigs(ldapConfig, ldapsConfig);
            server = new InMemoryDirectoryServer(serverConfig);
            server.startListening();
        } catch (LDAPException e) {
            fail(e.getMessage());
        }

The message tells me I need to add some kind of handler to the server, but I cannot find an example in the docs I've skimmed.