I'm trying to add an external URL to a button, but when I click it, it directs to an internal link ending with undefined

I have set up a channel with an url entry, all looks fine when I check the code, the buttons’ link is there, but when I click the button instead of going to the page what is set from the url entry, it directs the browser to mydomain.com/undefined I’ve checked the channel settings, tried to switch on and off the “render urls and email addresses as links” but nothing happened with either on or off.

When I add the same url entry to a normal <a> link, it works and directs the browser to the correct address.

What am I missing here?