diff --git a/lib/fluent/test/driver/base.rb b/lib/fluent/test/driver/base.rb
index 4a45d3b6a1..d087f72dcd 100644
--- a/lib/fluent/test/driver/base.rb
+++ b/lib/fluent/test/driver/base.rb
@@ -31,7 +31,9 @@ def initialize(klass, opts: {}, &block)
             if block
               # Create new class for test w/ overwritten methods
               #   klass.dup is worse because its ancestors does NOT include original class name
+              klass_name = klass.name
               klass = Class.new(klass)
+              klass.define_singleton_method("name") { klass_name }
               klass.module_eval(&block)
             end
             @instance = klass.new