Browser Caching is not working properly

I’m getting this error in Hummingbird:
Browser Caching is not working properly:
- Your server may not have the "expires" module enabled (mod_expires for Apache, ngx_http_headers_module for NGINX)
- Another plugin may be interfering with the configuration
If re-checking and restarting does not resolve, please check with your host or open a support ticket with us.

I enabled mod_expires for Apache and restarted the server, but didn't seem to make a difference.