[Zope] IOError: [Errno 9] Bad file descriptor

I have an external method that uses reportlab to make The method takes one argument, a Databse key, does a lookup of the database record for that key and then makes a PDF file using reportlab and writes it to the local HD of the zope server. The method works well outside of Zope, but called from Zope, it only works every second time (exactly). The first call will work, but the second will fail with: Error Type: IOError Error Value: [Errno 9] Bad file descriptor then the third call work work, etc. - any ideas? - Zope 2.7.0 on windows 2000 Server.