diff --git a/tool/mkconfig.rb b/tool/mkconfig.rb
index 6e23af5185..120b90850d 100755
--- a/tool/mkconfig.rb
+++ b/tool/mkconfig.rb
@@ -340,7 +340,6 @@ print <<EOS
     RbConfig::expand(val)
   end
 
-  # :nodoc:
   # call-seq:
   #
   #   RbConfig.fire_update!(key, val)               -> array
@@ -356,7 +355,7 @@ print <<EOS
   #   RbConfig::CONFIG.values_at("CC", "LDSHARED")          # => ["gcc-8", "gcc-8 -shared"]
   #
   # returns updated keys list, or +nil+ if nothing changed.
-  def RbConfig.fire_update!(key, val, mkconf = MAKEFILE_CONFIG, conf = CONFIG)
+  def RbConfig.fire_update!(key, val, mkconf = MAKEFILE_CONFIG, conf = CONFIG) # :nodoc:
     return if mkconf[key] == val
     mkconf[key] = val
     keys = [key]