ibtool gives error results when generating strings file

What steps will reproduce the problem?
1. cd trunk/; ibtool --generate-strings-file test.strings 
English.lproj/Preferences.nib
2. error occurs

What is the expected output? What do you see instead?
I should see a file test.strings generated without error reported, but instead 
of this, I see
error of "ibtool failed with exception: *** -[NSNull length]: unrecognized 
selector sent to instance 
0xa02c7020"

reference:
http://developer.apple.com/documentation/MacOSX/Conceptual/BPInternational/Artic
les/Localizin
gInterfaces.html

Original issue reported on code.google.com by mjhs...@gmail.com on 13 Jan 2009 at 9:33

Robert-T9911 / nally

ibtool gives error results when generating strings file #10