diff --git a/pdfrw/pdfwriter.py b/pdfrw/pdfwriter.py
index 3c887ba..cb884cb 100755
--- a/pdfrw/pdfwriter.py
+++ b/pdfrw/pdfwriter.py
@@ -138,7 +138,7 @@ def format_obj(obj):
                     if compress and obj.stream:
                         do_compress([obj])
                     pairs = sorted((getattr(x, 'encoded', None) or x, y)
-                                   for (x, y) in obj.iteritems())
+                                   for (x, y) in iteritems(obj))
                     myarray = []
                     for key, value in pairs:
                         myarray.append(key)
@@ -262,6 +262,7 @@ def __init__(self, fname=None, version='1.3', compress=False, **kwargs):
                                      "on PdfWriter instance" % name)
                 setattr(self, name, value)
 
+        self.Pages = None
         self.pagearray = PdfArray()
         self.killobj = {}
 
@@ -310,14 +311,14 @@ def _get_trailer(self):
             self.make_canonical()
 
         # Create the basic object structure of the PDF file
+        pages = self.Pages or IndirectPdfDict()
+        pages.Type = PdfName.Pages
+        pages.Count = PdfObject(len(self.pagearray))
+        pages.Kids = self.pagearray
         trailer = PdfDict(
             Root=IndirectPdfDict(
                 Type=PdfName.Catalog,
-                Pages=IndirectPdfDict(
-                    Type=PdfName.Pages,
-                    Count=PdfObject(len(self.pagearray)),
-                    Kids=self.pagearray
-                )
+                Pages=pages,
             )
         )
         # Make all the pages point back to the page dictionary and
diff --git a/tests/test_examples.py b/tests/test_examples.py
index 6871b80..aac2d00 100755
--- a/tests/test_examples.py
+++ b/tests/test_examples.py
@@ -94,7 +94,7 @@ def do_test(self, params, prev_results=[''], scrub=False):
                     os.remove(dstf)
                 if scrub and os.path.exists(scrub):
                     os.remove(scrub)
-                subprocess.call(params)
+                subprocess.check_output(params)
                 if scrub:
                     PdfWriter(dstf).addpages(PdfReader(scrub).pages).write()
             with open(dstf, 'rb') as f:
@@ -109,7 +109,7 @@ def do_test(self, params, prev_results=[''], scrub=False):
             if expects:
                 if len(expects) == 1:
                     expects, = expects
-                    self.assertEqual(hash, expects)
+                    self.assertEqual(hash, expects, '{} file does not have the expected MD5 hash'.format(dstf))
                 else:
                     self.assertIn(hash, expects)
                 result = 'pass'
diff --git a/tests/test_roundtrip.py b/tests/test_roundtrip.py
index 2e097b6..62bdcc9 100755
--- a/tests/test_roundtrip.py
+++ b/tests/test_roundtrip.py
@@ -91,7 +91,7 @@ def roundtrip(self, testname, basename, srcf, decompress=False,
             if expects:
                 if len(expects) == 1:
                     expects, = expects
-                    self.assertEqual(hash, expects)
+                    self.assertEqual(hash, expects, '{} file does not have the expected MD5 hash'.format(dstf))
                 else:
                     self.assertIn(hash, expects)
                 result = 'pass'