Prestashop 500 server error when required fields left blank

I'm using Prestashop 1.6.1.17.

I added some new required fields to the registration form (rut_required and telefono_required), by following this tutorial:

http://prestacraft.com/adding-new-fields-to-the-registration-form/#

It works well when filling the registration form.

However, when I login to check an old customer account information (with blank values for those new fields) and I go to "My account" and click "My personal information" I get a 500 server error. I could fix it by filling those fields directly from the database, but I want to be able to fill them from this form.

On the other hand, when I edit the same old customer account information from the back office (BackOffice > Customers > Edit), the page loads correctly and I can see those new fields with blank values.

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