Some phones get Base64 errors

[thinkfr33ly]

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?