add_filter ONLY for is_single

I’ve written a plugin to add a dropcap to the first letter of all my posts. At present it applies to pages as well as posts, I only want it to display on Posts.

i tried this

if (is_single()) {
add_filter(‘the_content’, ‘drop_cap’);
}

in the plugin, but obviously that doesn’t work. is there a way to do this that i’m just not seeing? thanks for any help!!