sorry for inconvenience, it was my mistake

db/schema/ser.xml

@@ -91,9 +91,7 @@
          an error if they are not present, although it
          would produce a warning
       -->
-<!-- It produces an error, not warning!
     <xi:include href="extra.xml" xpointer="xpointer(data/table)">
 	<xi:fallback/>
     </xi:include>
--->
 </database>