diff --git a/NEWS b/NEWS
index 08db61d58eb..11cf6cd0214 100644
--- a/NEWS
+++ b/NEWS
@@ -39,6 +39,7 @@ 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)
 
 - 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 a8e3b035d14..188806564c6 100644
--- a/ext/dom/node.c
+++ b/ext/dom/node.c
@@ -1093,6 +1093,13 @@ 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>