Server config send s a request that looks like this.

{"method":"config_mod","params":[[null],{"all":true,"setattr":["cn=ipaConfig"],"addattr":[],"rights":true,"ipacertificatesubjectbase":"O=IPA14.AYOUNG.BOSTON.DEVEL.REDHAT.COM","ipadefaultloginshell":"/bin/sh","ipadefaultprimarygroup":"ipausers","ipagroupsearchfields":"cn,description","ipahomesrootdir":"/home","ipamaxusernamelength":"32","ipamigrationenabled":"FALSE","ipasearchrecordslimit":"101","ipasearchtimelimit":"2","ipausersearchfields":"uid,givenname,sn,telephonenumber,ou,title"}],"id":52}

The null value is causing it to error out with an error like this:

"command 'config_mod' takes no arguments",

Removing the null works/