Vista - Net.WebRequest.Create objects GetResponse method returns -1.. why?

I have an small app. that uses WebRequest class in a scholar way like:

dim imgWeReq=system.net.WebRequest.Create(some_url_string")
dim imgWebRes as net.HttpWebResponse

imgWebRes= imgWebReq.getResponse


but, sometimes without any reason, ContentLength property of imgWebRes
equals -1.
but it downloads file successfully...

why I cannot read that property sometimes?
how could i correct this?

tnx