diff --git a/middleman-core/lib/middleman-core/sources/source_watcher.rb b/middleman-core/lib/middleman-core/sources/source_watcher.rb
index 07d7ff62..88d75f3f 100644
--- a/middleman-core/lib/middleman-core/sources/source_watcher.rb
+++ b/middleman-core/lib/middleman-core/sources/source_watcher.rb
@@ -199,7 +199,7 @@ module Middleman
     Contract ArrayOf[Pathname]
     def poll_once!
       updated = ::Middleman::Util.all_files_under(@directory.to_s, &method(:should_not_recurse?))
-      removed = @files.keys.reject { |p| updated.include?(p) }
+      removed = @files.keys - updated
 
       result = update(updated, removed)