gzip is not working properly

Trying to enable GZIP compression for text/HTML using Hummingbird, but I'm getting this error:
gzip is not working properly:
- Your server may not have the "deflate" module enabled (mod_deflate for Apache, ngx_http_gzip_module for NGINX)
- Another plugin may be interfering with the configuration

It shows as enabled for text/javascript and text/css, but disabled for text/html. That seems odd--seems like if it was working, it would work for all of them!