diff --git a/src/calibre/ebooks/oeb/polish/check/css.py b/src/calibre/ebooks/oeb/polish/check/css.py
index f7d7ea3b68..efce567d32 100644
--- a/src/calibre/ebooks/oeb/polish/check/css.py
+++ b/src/calibre/ebooks/oeb/polish/check/css.py
@@ -222,7 +222,12 @@ class Pool(object):
             self.working = False
 
     def shutdown(self):
-        tuple(map(sip.delete, self.workers))
+
+        def safe_delete(x):
+            if not sip.isdeleted(x):
+                sip.delete(x)
+
+        tuple(map(safe_delete, self.workers))
         self.workers = []