diff --git a/spec/acceptance_spec.rb b/spec/acceptance_spec.rb
index cf2e95c..c768a4a 100644
--- a/spec/acceptance_spec.rb
+++ b/spec/acceptance_spec.rb
@@ -111,7 +111,7 @@ describe MailCatcher do
     plain_tab_element.click
 
     iframe_element.displayed?.must_equal true
-    iframe_element.attribute(:src).must_match /\.plain\Z/
+    iframe_element.attribute(:src).must_match(/\.plain\Z/)
 
     selenium.switch_to.frame(iframe_element)