diff --git a/activerecord/lib/active_record/validations/uniqueness.rb b/activerecord/lib/active_record/validations/uniqueness.rb
index 5700350cf915af663750c059166e04dc16cd8e3f..2a4a70cd59a755973e2e4f243b928152d88e7c0a 100644
--- a/activerecord/lib/active_record/validations/uniqueness.rb
+++ b/activerecord/lib/active_record/validations/uniqueness.rb
@@ -33,7 +33,7 @@ def validate_each(record, attribute, value)
         end
 
         relation = finder_class.unscoped.where(relation)
-        relation = relation.merge(options[:conditions]) if options[:conditions]
+        relation.merge!(options[:conditions]) if options[:conditions]
 
         if relation.exists?
           error_options = options.except(:case_sensitive, :scope, :conditions)