Why does this really simple script fail to add a MySQL record?

Yes it does...I get this

[2016-04-23 19:14:17] local.DEBUG: Resource event: user.register.post.pre_process
[2016-04-23 19:14:17] local.DEBUG: Resource event: user.register.post.post_process
[2016-04-23 19:14:18] local.DEBUG: Resource event: Hiyer.table.{tablename}.post.pre_process
[2016-04-23 19:14:18] local.DEBUG: Resource event: Hiyer.table.Users.post.preprocess
[2016-04-23 19:14:18] local.DEBUG: Resource event: Hiyer.table.Users.post.postprocess
[2016-04-23 19:14:18] local.DEBUG: Resource event: Hiyer.table.{tablename}.post.post_process
[2016-04-23 19:14:18] local.INFO: * Script "user.register.post.post_process" output:
object(Object)#1176534077 (1) {
["success"] =>
bool(true)
}
object(Object)#591631922 (5) {
["email"] =>
string(15) "max4@tester.com"
["first_name"] =>
string(3) "Max"
["last_name"] =>
string(6) "Thomas"
["display_name"] =>
string(4) "Mr T"
["new_password"] =>
string(6) "123456"
}
NULL

I used this in the script for logging

var_dump(event.response.content);
var_dump(event.request.payload);
var_dump(event.request.payload.resource);

Is the payload as we would expect...for instance I do not have a a field in the database for display_name but I did not think I was asking for one.Now I look at the script, how does DF know which fields to map to the db?