Application.OpenURL not working

I'm trying to get my app to link to my other app using a popup. I have a UI button with the following code linked to its OnClick event:

 public void PopupClick(){
         Application.OpenURL ("https://play.google.com/store/apps/details?id=com.UltimateStudios.MuffinClicker");
     }

I don't know why, but in the editor and on my android phone when I click the button it doesnt open the URL.