No output from pdftk

I am trying to add a background to a PDF with pdftk:

$ pdftk ./sources/Skills.pdf background ./Background.pdf output test.pdf verbose
Command Line Data is valid.

Input PDF Filenames & Passwords in Order
( <filename>[, <password>] ) 
   ./sources/Skills.pdf

The operation to be performed: 
   filter - Apply 'filters' to a single, input PDF based on output args.
      (When the operation is omitted, this is the default.)

The output file will be named:
   test.pdf

Output PDF encryption settings:
   Output PDF will not be encrypted.

No compression or uncompression being performed on output.

Creating Output ...

The process is then stuck. It created the output file but without any content.

$ ls -l  test.pdf 
-rw-r--r--@ 1 corti  macports  0 Aug  9 14:27 test.pdf

I am using pdftk 2.02

pdftk 2.02 a Handy Tool for Manipulating PDF Documents Copyright (c)
2003-13 Steward and Lee, LLC – Please Visit: www.pdftk.com This is
free software; see the source code for copying conditions. There is NO
warranty, not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR
PURPOSE.

Any idea what could be the problem? Are there other (free) alternatives to add a PDF as a background for a second PDF?