Extending another Class in an Extension

An extension that I wrote is blocking the backend Add-Ons->Extensions page from loading. All I get when I try to visit the Extensions page is a blank white screen without any error messages whatsoever.

After playing around with my extension a bit, I discovered that this has something to do with the fact that my extension extends a class from another add-on.

Here’s the code that I have:

class Store_order_status_ext extends Store_orders_model { ... }

As soon as I change that to this, the Extensions page loads fine again:

class Store_order_status_ext { ... }

The extension itself works perfectly, but I can’t figure out why that “extends Store_orders_model” part is preventing the backend Extensions page from loading. I’m completely stumped. Any ideas? Thanks!