diff --git a/middleman-core/lib/middleman-core/sitemap/extensions/ignores.rb b/middleman-core/lib/middleman-core/sitemap/extensions/ignores.rb
index 9c2ea2db..2c9feb68 100644
--- a/middleman-core/lib/middleman-core/sitemap/extensions/ignores.rb
+++ b/middleman-core/lib/middleman-core/sitemap/extensions/ignores.rb
@@ -68,11 +68,14 @@ module Middleman
                 @ignored_callbacks << Proc.new {|p| File.fnmatch(path_clean, p) }
               else
                 # Add a specific-path ignore unless that path is already covered
-                @ignored_callbacks << Proc.new {|p| p == path_clean } unless ignored?(path_clean)
+                return if ignored?(path_clean)
+                @ignored_callbacks << Proc.new {|p| p == path_clean }
               end
             elsif block_given?
               @ignored_callbacks << block
             end
+
+            @app.sitemap.invalidate_resources_not_ignored_cache!
           end
 
           # Whether a path is ignored
diff --git a/middleman-core/lib/middleman-core/sitemap/store.rb b/middleman-core/lib/middleman-core/sitemap/store.rb
index d785e289..8a0ce73c 100644
--- a/middleman-core/lib/middleman-core/sitemap/store.rb
+++ b/middleman-core/lib/middleman-core/sitemap/store.rb
@@ -90,11 +90,17 @@ module Middleman
           if include_ignored
             @resources
           else
-            @resources.reject(&:ignored?)
+            @resources_not_ignored ||= @resources.reject(&:ignored?)
           end
         end
       end
 
+      # Invalidate our cached view of resource that are not ingnored. If your extension
+      # adds ways to ignore files, you should call this to make sure #resources works right.
+      def invalidate_resources_not_ignored_cache!
+        @resources_not_ignored = nil
+      end
+
       # Register a handler to provide metadata on a file path
       # @param [Regexp] matcher
       # @return [Array<Array<Proc, Regexp>>]
@@ -213,6 +219,8 @@ module Middleman
 
             newres
           end
+
+          invalidate_resources_not_ignored_cache!
         end
       end