diff --git a/src/calibre/ebooks/oeb/iterator/extract_fonts.py b/src/calibre/ebooks/oeb/iterator/extract_fonts.py
index fd907f4e37..ef44a8bb08 100644
--- a/src/calibre/ebooks/oeb/iterator/extract_fonts.py
+++ b/src/calibre/ebooks/oeb/iterator/extract_fonts.py
@@ -45,7 +45,7 @@ class FamilyMap(dict):
         import cssutils
         cssutils.log.setLevel(logging.ERROR)
         try:
-            sheet = cssutils.parseString(css)
+            sheet = cssutils.parseString(css, validate=False)
         except:
             return
         for rule in sheet.cssRules: