diff --git a/src/calibre/gui2/__init__.py b/src/calibre/gui2/__init__.py
index 668c876ca7..2b99bcb2c2 100644
--- a/src/calibre/gui2/__init__.py
+++ b/src/calibre/gui2/__init__.py
@@ -132,7 +132,7 @@ def _config(): # {{{
     c.add_opt('LRF_ebook_viewer_options', default=None,
               help=_('Options for the LRF ebook viewer'))
     c.add_opt('internally_viewed_formats', default=['LRF', 'EPUB', 'LIT',
-        'MOBI', 'PRC', 'AZW', 'HTML', 'FB2', 'PDB', 'RB', 'SNB'],
+        'MOBI', 'PRC', 'AZW', 'HTML', 'FB2', 'PDB', 'RB', 'SNB', 'HTMLZ'],
               help=_('Formats that are viewed using the internal viewer'))
     c.add_opt('column_map', default=ALL_COLUMNS,
               help=_('Columns to be displayed in the book list'))