diff --git a/src/calibre/gui2/tweak_book/preview.py b/src/calibre/gui2/tweak_book/preview.py
index 3becb5849f..fea102a977 100644
--- a/src/calibre/gui2/tweak_book/preview.py
+++ b/src/calibre/gui2/tweak_book/preview.py
@@ -172,7 +172,7 @@ class NetworkReply(QNetworkReply):
         if data is None:
             return QTimer.singleShot(10, self.check_for_parse)
         self.__data = data
-        self.setHeader(QNetworkRequest.ContentTypeHeader, 'text/html; charset=utf-8')
+        self.setHeader(QNetworkRequest.ContentTypeHeader, 'application/xhtml+xml; charset=utf-8')
         self.setHeader(QNetworkRequest.ContentLengthHeader, len(self.__data))
         self.finalize_reply()