diff --git a/ext/dom/node.c b/ext/dom/node.c
index 83404d0f598d..bbe8311147f9 100644
--- a/ext/dom/node.c
+++ b/ext/dom/node.c
@@ -2263,7 +2263,11 @@ static void dom_canonicalization(INTERNAL_FUNCTION_PARAMETERS, int mode) /* {{{
 		}
 
 		zend_hash_init(&links, 0, NULL, NULL, false);
-		dom_relink_ns_decls(&links, xmlDocGetRootElement(docp));
+		xmlNodePtr root_element = xmlDocGetRootElement(docp);
+
+		if (root_element) {
+			dom_relink_ns_decls(&links, root_element);
+		}
 	} else if (!docp) {
 		/* Note: not triggerable with modern DOM */
 		zend_throw_error(NULL, "Node must be associated with a document");
diff --git a/ext/dom/tests/modern/html/gh21688.phpt b/ext/dom/tests/modern/html/gh21688.phpt
new file mode 100644
index 000000000000..ddfb4d326c63
--- /dev/null
+++ b/ext/dom/tests/modern/html/gh21688.phpt
@@ -0,0 +1,13 @@
+--TEST--
+GH-21688 (SEGV in C14N on empty HTMLDocument)
+--CREDITS--
+YuanchengJiang
+--EXTENSIONS--
+dom
+--FILE--
+<?php
+$dom = Dom\HTMLDocument::createEmpty();
+var_dump($dom->C14N());
+?>
+--EXPECT--
+string(0) ""