diff --git a/lib/DOM/Document.php b/lib/DOM/Document.php
index a5dec08..b13bb3e 100644
--- a/lib/DOM/Document.php
+++ b/lib/DOM/Document.php
@@ -100,7 +100,7 @@ class Document extends \DOMDocument {
             $namespaceURI = trim($namespaceURI);
             $namespaceURI = ($namespaceURI === Parser::HTML_NAMESPACE) ? null : $namespaceURI;
         }
-        $qualifiedName = trim($qualifiedName);
+        $qualifiedName = ($namespaceURI === null) ? strtolower(trim($qualifiedName)) : trim($qualifiedName);
 
         try {
             if ($qualifiedName !== 'template' || $namespaceURI !== null) {
diff --git a/tests/cases/TestDOM.php b/tests/cases/TestDOM.php
index bab5496..cfdad31 100644
--- a/tests/cases/TestDOM.php
+++ b/tests/cases/TestDOM.php
@@ -18,24 +18,30 @@ use MensBeam\HTML\Document;
  */
 class TestDOM extends \PHPUnit\Framework\TestCase {
     /** @dataProvider provideNamespacedElements */
-    public function testCreateNamespacedElements(?string $ns, string $name, string $local, string $prefix): void {
+    public function testCreateNamespacedElements(?string $nsIn, string $nameIn, ?string $nsOut, string $local, string $prefix): void {
         $d = new Document;
-        $e = $d->createElementNS($ns, $name);
-        $this->assertSame($ns, $e->namespaceURI);
+        $e = $d->createElementNS($nsIn, $nameIn);
+        $this->assertSame($nsOut, $e->namespaceURI);
         $this->assertSame($local, $e->localName);
         $this->assertSame($prefix, $e->prefix);
     }
 
     public function provideNamespacedElements(): iterable {
         return [
-            [null,                         "test",           "test",            ""],
-            [null,                         "test:test",      "testU00003Atest", ""],
-            ["http://www.w3.org/2000/svg", "svg",            "svg",             ""],
-            ["http://www.w3.org/2000/svg", "svg:svg",        "svg",             "svg"],
-            ["fake_ns",                    "test:test",      "test",            "test"],
-            ["fake_ns",                    "test:test:test", "testU00003Atest", "test"],
-            ["fake_ns",                    "te st:test",     "test",            "teU000020st"],
-            [null,                         "9",              "U000039",         ""],
+            [null,                                 "test",           null,                                 "test",            ""],
+            [null,                                 "test:test",      null,                                 "testU00003Atest", ""],
+            ["http://www.w3.org/2000/svg",         "svg",            "http://www.w3.org/2000/svg",         "svg",             ""],
+            ["http://www.w3.org/2000/svg",         "svg:svg",        "http://www.w3.org/2000/svg",         "svg",             "svg"],
+            ["fake_ns",                            "test:test",      "fake_ns",                            "test",            "test"],
+            ["fake_ns",                            "test:test:test", "fake_ns",                            "testU00003Atest", "test"],
+            ["fake_ns",                            "te st:test",     "fake_ns",                            "test",            "teU000020st"],
+            [null,                                 "9",              null,                                 "U000039",         ""],
+            ["http://www.w3.org/1999/xhtml",       "test",           null,                                 "test",            ""],
+            ["http://www.w3.org/1999/xhtml",       "TEST",           null,                                 "test",            ""],
+            [null,                                 "TEST",           null,                                 "test",            ""],
+            ["fake_ns",                            "TEST",           "fake_ns",                            "TEST",            ""],
+            ["http://www.w3.org/2000/svg",         "TEST",           "http://www.w3.org/2000/svg",         "TEST",            ""],
+            ["http://www.w3.org/1998/Math/MathML", "TEST",           "http://www.w3.org/1998/Math/MathML", "TEST",            ""],
         ];
     }
 }