We have a WAP site using ASP.NET mobile controls. Everything works well
except for when two phone models attempt to access the site. When these
particular phones access the site they get the following exception:

System.FormatException: Invalid length for a Base-64 char array.
at System.Convert.FromBase64String(String s)
at System.Web.UI.LosFormatter.Deserialize(String input)
at
System.Web.UI.MobileControls.MobilePage.LoadPageStateFromPersistenceMedium()
at System.Web.UI.Page.LoadPageViewState()
at System.Web.UI.Page.ProcessRequestMain()

These phones are the Motorola 8720 andthe Samsung SGH 427M.

There may be additional phones that cause this error, but we have yet
to see them hit the site.

Why does this occur and is there any way to fix it?

Some additional information...

It seems that this is only occuring on POSTs, although this is very
confusing since the only page on our WAP site that does a POST (form
submit) is a page that usually doesn't cause the problem.

This kind of makes sense as far as the error goes since the viewstate
would only ever be sent back to the server on a POST anyway.