Only variables should be passed by reference in file_styles_strstr()

I obtain the following error message when adding a new image to a banner. I don't see any malfunction, but the warning hapen in the block configuration window and is display once for each image of the banner.

Strict warning: Only variables should be passed by reference in file_styles_strstr() (line 64 of path_to_drupal\sites\all\modules\styles\contrib\file_styles\file_styles.module).

Media 7.x-1.0-beta3
Styles/File Styles 7.x-2.0-alpha5
Rotating banner 7.x-7.x-dev