diff --git a/epub.js b/epub.js
index 6993488..d955753 100644
--- a/epub.js
+++ b/epub.js
@@ -673,8 +673,9 @@ class Loader {
         if ([MIME.XHTML, MIME.HTML, MIME.SVG].includes(mediaType)) {
             let doc = new DOMParser().parseFromString(str, mediaType)
             // change to HTML if it's not valid XHTML
-            if (mediaType === MIME.XHTML && doc.querySelector('parsererror')) {
-                console.warn(doc.querySelector('parsererror').innerText)
+            if (mediaType === MIME.XHTML && (doc.querySelector('parsererror')
+            || !doc.documentElement?.namespaceURI)) {
+                console.warn(doc.querySelector('parsererror')?.innerText ?? 'Invalid XHTML')
                 item.mediaType = MIME.HTML
                 doc = new DOMParser().parseFromString(str, item.mediaType)
             }
diff --git a/mobi.js b/mobi.js
index 31ff1dc..116477d 100644
--- a/mobi.js
+++ b/mobi.js
@@ -1154,7 +1154,7 @@ class KF8 {
 
         // by default, type is XHTML; change to HTML if it's not valid XHTML
         let doc = this.parser.parseFromString(replaced, this.#type)
-        if (doc.querySelector('parsererror')) {
+        if (doc.querySelector('parsererror') || !doc.documentElement?.namespaceURI) {
             this.#type = MIME.HTML
             doc = this.parser.parseFromString(replaced, this.#type)
         }