Cloudflare Pages - value for environment variable not found

This is going to sound like a very basic question (apologies in advance if so) - but we just deployed a static client (AngularJS) to Cloudflare Pages. I’ve already set a custom domain, but it’s in “Verifying” state. When we deploy, it seems to work, but when I load the site (at the custom domain) - I see an error like this:

Uncaught Error: API_HOST is not defined in .env

I definitely have an environment variable called API_HOST with a value in my environment variables.

What else could be wrong?