diff --git a/src/calibre/ebooks/oeb/polish/check/main.py b/src/calibre/ebooks/oeb/polish/check/main.py
index aa0e2f748c..256c57ac6e 100644
--- a/src/calibre/ebooks/oeb/polish/check/main.py
+++ b/src/calibre/ebooks/oeb/polish/check/main.py
@@ -52,6 +52,9 @@ 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))