diff --git a/foliator.py b/foliator.py
index eca5949..ef4baae 100755
--- a/foliator.py
+++ b/foliator.py
@@ -1,9 +1,10 @@
 #!/usr/bin/env python3
 
 import argparse
-from pypdf import PaperSize, PdfReader, PdfWriter, Transformation
-from pypdf.annotations import Line
+import numpy as np
+from pypdf import PaperSize, PdfReader, PdfWriter, Transformation, PageObject
 from pypdf.generic import ArrayObject, FloatObject, NameObject
+from pypdf.annotations import Line
 import math
 
 
@@ -106,7 +107,7 @@ def blit_to_A4(input_pdf, append_to=PdfWriter()):
             writer.add_annotation(front_page, midline)
 
         front = input_pdf.pages[k]
-        back = input_pdf.pages[k+1] if (k+1<L) else create_blank_page(None, PaperSize.A5.width, PaperSize.A5.height)
+        back = input_pdf.pages[k+1] if (k+1<L) else PageObject.create_blank_page(None, PaperSize.A5.width, PaperSize.A5.height)
 
         front.mediabox = front_page.mediabox
         back.mediabox  = back_page.mediabox
diff --git a/shell.nix b/shell.nix
index d917189..0f5ffee 100644
--- a/shell.nix
+++ b/shell.nix
@@ -6,6 +6,7 @@ pkgs.mkShell {
   packages = [
   (pkgs.python3.withPackages (pythonPackages: with python3Packages; [
     pypdf
+    numpy
   ]))
   ];
 }