Adding contacts via API doesn't import "Mailing Postal Code" and "Mailing Province" data

Adding contacts via API doesn't import "Mailing Postal Code" and "Mailing Province" data

The contacts are importing correctly except for these two fields.  Here is my code where I am building up the data I send to the API. It won't mean much except to illustrate my point :

 contact.Add(new KeyValuePair<string, string>("Mailing Street", customer.CombinedStreet));
 contact.Add(new KeyValuePair<string, string>("Mailing Province", customer.StateName));
 contact.Add(new KeyValuePair<string, string>("Mailing Postal Code", customer.PostCode));   
 contact.Add(new KeyValuePair<string, string>("Mailing City", customer.Locality));
 contact.Add(new KeyValuePair<string, string>("Mailing Country", customer.CountryName));  

When the data is sent to the API, "Mailing Street", "Mailing City" and "Mailing Country" all import correctly, but "Mailing Postal Code" and "Mailing Province" are blank.

I have checked the http payload being sent and the data is definitely there, so I don't think I have a bug at my end.

I suspect it is something to do with the name of the fields, perhaps the API is expecting a different name to that which is displayed when I view the contact fields list in the Customization screen in Zoho.

Below is a screenshot of the contact fields list :