diff --git a/src/calibre/gui2/tweak_book/jump_to_class.py b/src/calibre/gui2/tweak_book/jump_to_class.py
index 7c9867f50a..675670ac4e 100644
--- a/src/calibre/gui2/tweak_book/jump_to_class.py
+++ b/src/calibre/gui2/tweak_book/jump_to_class.py
@@ -5,11 +5,11 @@ from contextlib import suppress
 from typing import List, NamedTuple, Optional, Tuple
 
 from css_parser.css import CSSRule
-from css_selectors import Select, SelectorError
 
 from calibre.ebooks.oeb.parse_utils import barename
 from calibre.ebooks.oeb.polish.container import get_container
 from calibre.ebooks.oeb.polish.parsing import parse
+from css_selectors import Select, SelectorError
 
 
 class NoMatchingTagFound(KeyError):
@@ -116,7 +116,7 @@ def find_first_matching_rule(
             )
             if res is not None:
                 return res._replace(style_tag_address=(int(tag.get(lnum_attr)), ['style']))
-        elif tn == 'link' and tag.get('href') and tag.get('rel') == 'stylesheet':
+        elif tn == 'link' and tag.get('href') and tag.get('rel', '').lower() == 'stylesheet':
             sname = container.href_to_name(tag.get('href'), html_file_name)
             try:
                 sheet = container.parsed(sname)