diff --git a/lib/logstash/filters/mutate.rb b/lib/logstash/filters/mutate.rb
index b0693a4..0172348 100644
--- a/lib/logstash/filters/mutate.rb
+++ b/lib/logstash/filters/mutate.rb
@@ -508,9 +508,8 @@ def merge(event)
     @merge.each do |dest_field, added_fields|
       # When multiple calls, added_field is an array
 
-      dest_field_value = event.get(dest_field)
-
       Array(added_fields).each do |added_field|
+        dest_field_value = event.get(dest_field)
         added_field_value = event.get(added_field)
 
         if dest_field_value.is_a?(Hash) ^ added_field_value.is_a?(Hash)