C# - ViewState Is NULL In Page_Load But Not Postback?

I've got a weird problem with a NullReferenceException on a high traffic website my company hosts. The exceptions are logged with full stack-traces but I am unable to reproduce it.The exception occurs a couple of times a day, for different users, and it's a NullReferenceException thrown in the code block below:

protected void Page_Load(object sender, EventArgs e)
{
...

if (!Page.IsPostBack)
{
...
this.ViewState[StaticClass.StaticStringProperty] = StaticClass.StaticIntProperty; // this is the line the exception occurs on
...
}
}

The only place I can figure that a NullReferenceException would be thrown is if ViewState is NULL, but I've never known that and can't find any reason why this would be the case in a Page_Load that isn't a postback.StaticStringProperty and StaticIntProperty are both initialised, static properties of StaticClass.

StaticStringProperty is defined as:

public const string StaticStringProperty = "IdFromClient";

Does anyone know how this could happen, or any other reason why a NullReferenceException would be thrown on the above line?

EDIT.As requested, the full stack-trace for the error is as follows. Line 54 is the line I've highlighted above.

at MyCompany.MyApplication.Appliance.Page_Load(Object sender, EventArgs e) in C:Documents and SettingsshellamaMy DocumentsMyApplicationAppliance.aspx.cs:line 54
at System.Web.Util.CalliHelper.EventArgFunctionCaller(IntPtr fp, Object o, Object t, EventArgs e)[code]....