diff --git a/ext/dom/php_dom.c b/ext/dom/php_dom.c
index 6bc72e9f97940..2f2878d5e1521 100644
--- a/ext/dom/php_dom.c
+++ b/ext/dom/php_dom.c
@@ -1302,9 +1302,13 @@ int dom_hierarchy(xmlNodePtr parent, xmlNodePtr child)
 {
 	xmlNodePtr nodep;
 
-    if (parent == NULL || child == NULL || child->doc != parent->doc) {
-        return SUCCESS;
-    }
+	if (parent == NULL || child == NULL || child->doc != parent->doc) {
+		return SUCCESS;
+	}
+
+	if (child->type == XML_DOCUMENT_NODE) {
+		return FAILURE;
+	}
 
 	nodep = parent;
 
diff --git a/ext/dom/tests/bug66783.phpt b/ext/dom/tests/bug66783.phpt
new file mode 100644
index 0000000000000..98981a88f6e26
--- /dev/null
+++ b/ext/dom/tests/bug66783.phpt
@@ -0,0 +1,19 @@
+--TEST--
+Bug #66783 (UAF when appending DOMDocument to element)
+--SKIPIF--
+<?php
+if (!extension_loaded('dom')) die('skip dom extension not available');
+?>
+--FILE--
+<?php
+$doc = new DomDocument;
+$doc->loadXML('<root></root>');
+$e = $doc->createElement('e');
+try {
+    $e->appendChild($doc);
+} catch (DOMException $ex) {
+    echo $ex->getMessage(), PHP_EOL;
+}
+?>
+--EXPECTF--
+Hierarchy Request Error