Fix #1787

middleman-core/lib/middleman-core/extensions.rb

@@ -106,7 +106,7 @@ module Middleman
       # A flattened list of all extensions which are automatically activated
       # @return [Array<Symbol>] A list of extension names which are automatically activated.
       def auto_activated
-        @auto_activate.values.map(&:to_a).flat_map(&:name)
+        @auto_activate.values.map(&:to_a).flatten.map(&:name)
       end
 
       # @api private

middleman-core/lib/middleman-core/version.rb

@@ -1,5 +1,5 @@
 module Middleman
   # Current Version
   # @return [String]
-  VERSION = '4.1.0'.freeze unless const_defined?(:VERSION)
+  VERSION = '4.1.1'.freeze unless const_defined?(:VERSION)
 end