Git submodule update errors

I run this site using a WordPress Skeleton setup. WordPress runs in a sub-directory as a submodule.

When a WordPress update is released, as it was this-morning, I update my local repo and push it to the site. My understanding is, when I log onto the server I should be able to run the following to update:

git pull
git submodule update

Every time, I get the error fatal: reference is not a tree: followed by the commit’s hash and have to mess around on the server to get the update working.

Answers on Stack Overflow aren’t helping, so any tips? Treat me like an idiot.