diff --git a/lib/fluent/config/configure_proxy.rb b/lib/fluent/config/configure_proxy.rb
index 4db4fd4ef0..fbb6b63af8 100644
--- a/lib/fluent/config/configure_proxy.rb
+++ b/lib/fluent/config/configure_proxy.rb
@@ -274,7 +274,7 @@ def parameter_configuration(name, type = nil, **kwargs, &block)
         option_value_type!(name, opts, :deprecated, String)
         option_value_type!(name, opts, :obsoleted, String)
         if type == :enum
-          if !opts.has_key?(:list) || !opts[:list].is_a?(Array) || opts[:list].empty? || !opts[:list].all?{|v| v.is_a?(Symbol) }
+          if !opts.has_key?(:list) || !opts[:list].is_a?(Array) || opts[:list].empty? || !opts[:list].all?(Symbol)
             raise ArgumentError, "#{name}: enum parameter requires :list of Symbols"
           end
         end
diff --git a/lib/fluent/config/types.rb b/lib/fluent/config/types.rb
index 98e7d0d9d8..9c9ba89c8d 100644
--- a/lib/fluent/config/types.rb
+++ b/lib/fluent/config/types.rb
@@ -123,7 +123,7 @@ def self.enum_value(val, opts = {}, name = nil)
 
       s = val.to_sym
       list = opts[:list]
-      raise "Plugin BUG: config type 'enum' requires :list of symbols" unless list.is_a?(Array) && list.all?{|v| v.is_a? Symbol }
+      raise "Plugin BUG: config type 'enum' requires :list of symbols" unless list.is_a?(Array) && list.all?(Symbol)
       unless list.include?(s)
         raise ConfigError, "valid options are #{list.join(',')} but got #{val}"
       end