Subdirectory install error

I’m seeing the error

the theme directory “twentyfifteen” does not exist

for the twentyfifteen theme after trying a WordPress subdirectory install for the first time.

I can access the admin panel ok at mysite.com/wp/wp-admin but the home page is blank (Not a WSOD 500 error but a 200 OK)

I’ve enabled WP_DEBUG but can’t find anything useful in the logs. WordPress version is 4.1

UPDATE

I also have wp-content in the document root directory. My file structure looks like this

|-- composer.json
|-- composer.lock
|-- index.html
|-- index.php
|-- vendor
|-- wp
|-- wp-config.php
`-- wp-content

I had forgotten to define the location of wp-content in my wp-config.php :

define( 'WP_CONTENT_DIR', dirname(__FILE__) . '/wp-content' );
define( 'WP_CONTENT_URL', 'http://'.$_SERVER['SERVER_NAME'].'/wp-content' );
define( 'WP_PLUGIN_DIR', dirname(__FILE__) . '/wp-content/plugins' );