Exception in vb while checking null value

Hi,

I want 2 check null value .I use following code. But the if condition line showing exception.It is giving a warning message like Variable sessionstring is used before it has been assigned.A null exception may occur at run time

For Each sessitem In Session.Contents

If Not String.IsNullOrEmpty(sessionString) Then

sessionString = sessionString & """" & sessitem & "|" & Session(sessitem).ToString() & """;"

End If

Next

any Idea.