VB: Occasional Error when Creating a String

Problem is, if I create a new string:
dim strTemp as string

It works consistently for me in the office, but occasionally a customer will get an object reference not set error the first time the string is used.

The solution is to create the string thusly:

dim strTemp as string = string.empty


The problem is, QA wants to be able to test the broken version in order to ensure that the problem is really fixed, but I have no friggin' clue how to create the circumstances that throw the error.

Any idea what causes the problem sometimes and not others?

edit: I forgot to mention that this is running on a webserver, not stand-alone.