block google app

[lists at lazygranch.com]

I would like to block the google app from directly downloading images. 

access.log:

200 186.155.157.9 - - [20/Jun/2017:00:35:47 +0000] "GET /images/photo.jpg HTTP/1.1" 334052 "-" "com.google.GoogleMobile/28.0.0 iPad/9.3.5 hw/iPad2_5" "-"


My nginx code in the images location:

if ($http_referer ~* (com.google.GoogleMobile)) {
    return 403;
    }

So what I am doing wrong?