Controls Property Not Set

I have created a custom control and one of the properties of it is a Rollover Image.
Here is the property

VB Code:

<DefaultValue(""), _
Description("This will set the rollover image of the control."), _
Editor(GetType(System.Web.UI.Design.ImageUrlEditor), _
GetType(System.Drawing.Design.UITypeEditor))> _
Public Property RolloverImage() As String
   Get
      mstrRolloverImage = ViewState("RolloverImage").ToString
      If mstrRolloverImage.Length = 0 Then
         Return mstrRolloverImage.Empty
      Else
         Return mstrRolloverImage
      End If
   End Get
   Set(ByVal Value As String)
      mstrRolloverImage = Value
      ViewState("RolloverImage") = mstrRolloverImage
   End Set
End Property

The problem occurs when I compile the control, add it to my toolbox, then drop it on my page...The Rollover Image property displays "Object reference not set to an instance of an object"...but I'm not sure why???

Anyone have any ideas how to fix this?