diff --git a/src/calibre/ebooks/oeb/polish/check/main.py b/src/calibre/ebooks/oeb/polish/check/main.py
index 256c57ac6e..ec4983374a 100644
--- a/src/calibre/ebooks/oeb/polish/check/main.py
+++ b/src/calibre/ebooks/oeb/polish/check/main.py
@@ -45,6 +45,9 @@ def run_checks(container):
     errors.extend(run_checkers(check_xml_parsing, html_items))
     errors.extend(run_checkers(check_raster_images, raster_images))
 
+    if errors:
+        return errors
+
     # cssutils is not thread safe
     for name, mt, raw in stylesheets:
         if not raw:
@@ -52,9 +55,6 @@ def run_checks(container):
             continue
         errors.extend(check_css_parsing(name, raw))
 
-    if errors:
-        return errors
-
     for name, mt, raw in html_items + xml_items:
         errors.extend(check_encoding_declarations(name, container))