Hello i am getting this error message :

Object reference not set to an instance of an object. Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.NullReferenceException: Object reference not set to an instance of an object.

Source Error:

Line 7:  
Line 8:      Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
Line 9:          UserIdValue.Text = Membership.GetUser().ProviderUserKey.ToString()
Line 10:         cannotUploadImageMessage.Visible = False
Line 11:     End Sub

can someone help me with this?