diff --git a/NEWS b/NEWS
index 1aaca5dc75b..1ccb911b779 100644
--- a/NEWS
+++ b/NEWS
@@ -41,6 +41,8 @@ PHP                                                                        NEWS
   . Fixed bug GH-16533 (Segfault when adding attribute to parent that is not
     an element). (nielsdos)
   . Fixed bug GH-16535 (UAF when using document as a child). (nielsdos)
+  . Fixed bug GH-16593 (Assertion failure in DOM->replaceChild). (nielsdos)
+  . Fixed bug GH-16595 (Another UAF in DOM -> cloneNode). (nielsdos)
 
 - EXIF:
   . Fixed bug GH-16409 (Segfault in exif_thumbnail when not dealing with a
diff --git a/ext/dom/node.c b/ext/dom/node.c
index beccd257e61..cc693cd8a36 100644
--- a/ext/dom/node.c
+++ b/ext/dom/node.c
@@ -990,7 +990,7 @@ Since:
 PHP_METHOD(DOMNode, insertBefore)
 {
 	zval *id, *node, *ref = NULL;
-	xmlNodePtr child, new_child, parentp, refp;
+	xmlNodePtr child, new_child, parentp, refp = NULL;
 	dom_object *intern, *childobj, *refpobj;
 	int ret, stricterror;
 
@@ -1015,6 +1015,14 @@ PHP_METHOD(DOMNode, insertBefore)
 		RETURN_FALSE;
 	}
 
+	if (ref != NULL) {
+		DOM_GET_OBJ(refp, ref, xmlNodePtr, refpobj);
+		if (refp->parent != parentp) {
+			php_dom_throw_error(NOT_FOUND_ERR, stricterror);
+			RETURN_FALSE;
+		}
+	}
+
 	if (child->doc == NULL && parentp->doc != NULL) {
 		dom_set_document_ref_pointers(child, intern->document);
 	}
@@ -1022,12 +1030,6 @@ PHP_METHOD(DOMNode, insertBefore)
 	php_libxml_invalidate_node_list_cache(intern->document);
 
 	if (ref != NULL) {
-		DOM_GET_OBJ(refp, ref, xmlNodePtr, refpobj);
-		if (refp->parent != parentp) {
-			php_dom_throw_error(NOT_FOUND_ERR, stricterror);
-			RETURN_FALSE;
-		}
-
 		if (child->parent != NULL) {
 			xmlUnlinkNode(child);
 		}
@@ -1173,6 +1175,13 @@ PHP_METHOD(DOMNode, replaceChild)
 		RETURN_FALSE;
 	}
 
+	/* This is already disallowed by libxml, but we should check it here to avoid
+	 * breaking assumptions and assertions. */
+	if ((oldchild->type == XML_ATTRIBUTE_NODE) != (newchild->type == XML_ATTRIBUTE_NODE)) {
+		php_dom_throw_error(HIERARCHY_REQUEST_ERR, stricterror);
+		RETURN_FALSE;
+	}
+
 	if (oldchild->parent != nodep) {
 		php_dom_throw_error(NOT_FOUND_ERR, stricterror);
 		RETURN_FALSE;
diff --git a/ext/dom/tests/gh16593.phpt b/ext/dom/tests/gh16593.phpt
new file mode 100644
index 00000000000..416d794a4f0
--- /dev/null
+++ b/ext/dom/tests/gh16593.phpt
@@ -0,0 +1,22 @@
+--TEST--
+GH-16593 (Assertion failure in DOM->replaceChild)
+--EXTENSIONS--
+dom
+--FILE--
+<?php
+
+$doc = new DOMDocument;
+$root = $doc->appendChild($doc->createElement('root'));
+$child = $root->appendChild($doc->createElement('child'));
+try {
+    $root->replaceChild($doc->createAttribute('foo'), $child);
+} catch (DOMException $e) {
+    echo $e->getMessage(), "\n";
+}
+echo $doc->saveXML();
+
+?>
+--EXPECT--
+Hierarchy Request Error
+<?xml version="1.0"?>
+<root><child/></root>