diff --git a/actionpack/lib/action_dispatch/routing/mapper.rb b/actionpack/lib/action_dispatch/routing/mapper.rb
index 0e8badc4987d01e982be77a4267756fff9852b57..5982dc862c455bb0def89a6fba66271ad27e2d07 100644
--- a/actionpack/lib/action_dispatch/routing/mapper.rb
+++ b/actionpack/lib/action_dispatch/routing/mapper.rb
@@ -1765,7 +1765,7 @@ def name_for_action(as, action) #:nodoc:
               # and return nil in case it isn't. Otherwise, we pass the invalid name
               # forward so the underlying router engine treats it and raises an exception.
               if as.nil?
-                candidate unless @set.named_routes.key?(candidate) || candidate !~ /\A[_a-z]/i
+                candidate unless candidate !~ /\A[_a-z]/i || @set.named_routes.key?(candidate)
               else
                 candidate
               end