Why is IndexIgnore used in htaccess not hiding files as it should?

I am attempting to hide certain files in a directory listing (index.php, style.css) by using IndexIgnore within my .htaccess file. The following code should but working but is not:

IndexIgnore .htaccess *.php *.css *.js

To be clear, when navigating to a directory, all files within that directory should appear except for the file type extensions listed above. No special software such as WordPress is installed on the subdomain which is hosting this file. Any ideas?

Thank you!