diff --git a/PyPDF2/pdf.py b/PyPDF2/pdf.py
index 547284f80..1d0e95011 100644
--- a/PyPDF2/pdf.py
+++ b/PyPDF2/pdf.py
@@ -1575,9 +1575,11 @@ def _flatten(self, pages=None, inherit=None, indirectRef=None):
         if inherit is None:
             inherit = dict()
         if pages is None:
-            self.flattenedPages = []
+            # Fix issue 327: set flattenedPages attribute only for
+            # decrypted file
             catalog = self.trailer["/Root"].getObject()
             pages = catalog["/Pages"].getObject()
+            self.flattenedPages = []
 
         t = "/Pages"
         if "/Type" in pages: