underscores theme mobile menu

I’m building a theme based on Underscores, and I’m having problems with the responsive menu.

When viewed on a mobile device the Primary Menu is replaced with a ‘Primary Menu’ button as desired, but it seems the button is wrapped in an anchor tag directed to the home page, so when the button is clicked the navigation is displayed for a moment but then the home page is loaded.

Any pointers on how I change that behaviour?