diff --git a/ext/nokogiri/xml_document.c b/ext/nokogiri/xml_document.c
index 1850849251..1d2119c80f 100644
--- a/ext/nokogiri/xml_document.c
+++ b/ext/nokogiri/xml_document.c
@@ -9,6 +9,9 @@ static int dealloc_node_i(xmlNodePtr key, xmlNodePtr node, xmlDocPtr doc)
   case XML_NAMESPACE_DECL:
     xmlFreeNs((xmlNsPtr)node);
     break;
+  case XML_DTD_NODE:
+    xmlFreeDtd((xmlDtdPtr)node);
+    break;
   default:
     if(node->parent == NULL) {
       xmlAddChild((xmlNodePtr)doc, node);