Theme path is wrong when using custom theme

The following error displays in a rather scary red box:

Fatal error: Theme at //sites/all/themes/mytheme/js/galleria/galleria.mytheme.js could not load, check theme path.

And the nothing relating to the slideshow displays.

If I modify line 135, which is currently:

    $theme_path = base_path() . '/'. $custom_theme_path;

..and simply remove the '/' . to form:

    $theme_path = base_path() . $custom_theme_path;

This fixes the problem.