Sometimes wp:post_name can be empty (e.g. when a post is still draft), in this case we make up an appropriate permalink_title that will be used as the filename. The importer can always rename the file later on, and at least the file is unlikely to have been overwritten by another draft on the same day.

lib/jekyll/migrators/wordpressdotcom.rb

@@ -19,6 +19,11 @@ module Jekyll
       (doc/:channel/:item).each do |item|
         title = item.at(:title).inner_text.strip
         permalink_title = item.at('wp:post_name').inner_text
+        # Fallback to "prettified" title if post_name is empty (can happen)
+	if permalink_title == ""
+		permalink_title = title.downcase.split.join('-')
+	end
+
         date = Time.parse(item.at('wp:post_date').inner_text)
         status = item.at('wp:status').inner_text
         type = item.at('wp:post_type').inner_text