diff --git a/lib/net/imap/config.rb b/lib/net/imap/config.rb
index 29c01bd4..642c88e7 100644
--- a/lib/net/imap/config.rb
+++ b/lib/net/imap/config.rb
@@ -631,6 +631,7 @@ def defaults_hash
         to_h.reject {|k,v| DEFAULT_TO_INHERIT.include?(k) }
       end
 
+      Struct   = AttrAccessors.struct
       @default = AttrVersionDefaults.compile_default!
       @global  = default.new
       AttrVersionDefaults.compile_version_defaults!
diff --git a/lib/net/imap/config/attr_accessors.rb b/lib/net/imap/config/attr_accessors.rb
index 8725428c..cef757e0 100644
--- a/lib/net/imap/config/attr_accessors.rb
+++ b/lib/net/imap/config/attr_accessors.rb
@@ -27,24 +27,23 @@ def self.included(mod)
 
         def self.attr_accessor(name) # :nodoc: internal API
           name = name.to_sym
+          raise ArgumentError, "already defined #{name}" if attributes.include?(name)
+          attributes << name
           def_delegators :data, name, :"#{name}="
         end
 
-        def self.attributes
-          instance_methods.grep(/=\z/).map { _1.to_s.delete_suffix("=").to_sym }
-        end
-        private_class_method :attributes
+        # An array of Config attribute names
+        singleton_class.attr_reader :attributes
+        @attributes = []
 
         def self.struct # :nodoc: internal API
-          unless defined?(self::Struct)
-            const_set :Struct, Struct.new(*attributes)
-          end
-          self::Struct
+          attributes.freeze
+          Struct.new(*attributes)
         end
 
         def initialize # :notnew:
           super()
-          @data = AttrAccessors.struct.new
+          @data = Config::Struct.new
         end
 
         # Freezes the internal attributes struct, in addition to +self+.
diff --git a/test/net/imap/test_config.rb b/test/net/imap/test_config.rb
index 6a35bff9..8e1ac569 100644
--- a/test/net/imap/test_config.rb
+++ b/test/net/imap/test_config.rb
@@ -363,7 +363,7 @@ def duck.to_r   = 1/11111
     expected = {
       debug: false, open_timeout: 30, idle_response_timeout: 5, sasl_ir: true,
     }
-    attributes = Config::AttrAccessors::Struct.members
+    attributes = Config::Struct.members
     default_hash = Config.default.to_h
     assert_equal expected, default_hash.slice(*expected.keys)
     assert_equal attributes, default_hash.keys