diff --git a/lib/logstash/filters/multiline.rb b/lib/logstash/filters/multiline.rb
index ae4b29222cb..985718eff95 100644
--- a/lib/logstash/filters/multiline.rb
+++ b/lib/logstash/filters/multiline.rb
@@ -5,6 +5,7 @@
 
 require "logstash/filters/base"
 require "logstash/namespace"
+require "set"
 
 # The multiline filter is for combining multiple events from a single source
 # into the same event.