diff --git a/functions/test/slack_test.rb b/functions/test/slack_test.rb
index 04ae82e72..6e85449b0 100644
--- a/functions/test/slack_test.rb
+++ b/functions/test/slack_test.rb
@@ -14,6 +14,7 @@
 
 require "helper"
 require "openssl"
+require "ostruct"
 
 describe "functions_slack" do
   include FunctionsFramework::Testing