EasyBlog subscribe gives blank pop-up

With EasyBlog if a user is already subscribed but attempts to subscribe again the submit button loads a blank pop-up box. I'm getting this same result on 3 sites with EasyBlog installed. Shouldn't there be text in the pop-up box notifying the user they are already subscribed? This happens with both the subscription module and the envelope icon in the toolbar.

This site is using:
Joomla 3.6.8
EasyBlog 5.1.15
EasyBlog subscribe module 5.1.15

When you find a solution I have at least 2 other websites with the same problem. Please explain the solution so I can apply it to the others.

Thank you,
Richard