diff --git a/test/drb/test_acl.rb b/test/drb/test_acl.rb
index 2ac905c1b8..ea7b32e76f 100644
--- a/test/drb/test_acl.rb
+++ b/test/drb/test_acl.rb
@@ -88,6 +88,8 @@ class ACLEntryTest < Test::Unit::TestCase
 
     assert_raise(IPAddr::InvalidPrefixError) {
       ACL::ACLEntry.new('192.168.0.0/33')
+    }
+    assert_raise(IPAddr::InvalidPrefixError) {
       ACL::ACLEntry.new('192.168.0.0/255.255.0.255')
     }
   end