coercing to Unicode: need string or buffer, NoneType found

[Jon Bowlas]

Hi listers,

I wrote this script in Zope some time ago and it worked for a while, but now 
I'm getting the following error:
TypeError: coercing to Unicode: need string or buffer, NoneType found

Here's my script:

results = context.module_retriever().tuples() # call to ZSQLMethod
converted = []
for result in results:
   result = list(result) # make a list from the tuple
   for i in range(len(result)):
    # for each element in the listified tuple, make decode to a
    # unicode from the latin-1 string
    result[i] = unicode(result[i], 'latin-1')
   converted.append(tuple(result)) # tuplify again
return converted

Where module_retriever is a simple Z SQL method that returns a module title 
and a module code.
So results = context.module_retriever().tuples() returns:
[('Developmental Neurobiology', 'ANAT2008'), ('Neuroanatomy', 
'ANAT2009'),('etc...', 'etc..'),..]So I was hoping someone might be able to 
identify what I've done wrong here. It is a little mistifying as to why it's 
suddenly stopped working though.CheersJon