file.tell() -- obscure IOError

[Uwe Mayer]

In article <MPG.1774c0f11ff79749989686 at news.rz.uni-karlsruhe.de>, 
merkosh at hadiko.de says...
> hi,
> 
> i have an object which conforms to the file protocoll. while calling the 
> self.file.tell()   method, i get the following error message:
> 
> >>> s1 == s2
> Traceback (most recent call last):
>   File "<stdin>", line 1, in ?
>   File "riff.py", line 153, in __cmp__
>     data1 = self.read(self.value.step)
>   File "riff.py", line 205, in read
>     maxsize = len(self) -self.tell()
>   File "riff.py", line 234, in tell
>     return self.file.tell() -self.value.start
> IOError: (0, 'Error')
> 

more info:
the tell() does not fail every time. what i do:
i compare two files by reading count bytes and calling cmp() on both of 
them until end of file or cmp() != 0 

the tell() failes for different numbers of 'count' bytes. f.e. 
count = 1     fails after 7 bytes
count = 5	works
count = 6 	fails right at the beginning

...?

any ideas?
Ciao
Uwe