Problem with field "<select>"

Hello,
I created a site : see here
and when i try to register with the registration form within the scope of the date of birth
I put months and then I select the field the day fades
Can you tell me the problem
and how I can solve it
see picture


Thank you

 

JS error in the file lib.js

daySelect.unload is not a function
[Break on this error] daySelect.unload(); 

Code (markup):

in line 421

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