Array Not Created Properly With Split Function

I am having trouble loading the result of a VBscript split function into an array. The script is supposed to take the user's login name (the form for this is name separated by underscores such as john_foo or lisa_wilson), split it at the underscores, and load first, middle and last name into an array. The problem I keep getting though, is that I am getting a type mismatch error. Here is the code:

<%dim LogName, NameArray(), FirstName, LastName, MiddleName, LastPage
'Get NT authorized login info
LogName = request.servervariables("logon_user")
NameArray = Split(LogName, "_")
for x = 0 to UBound(NameArray)
response.write NameArray(x) & "<br>"
next
%>

The FirstName, MiddleName, LastName vars are supposed to take the array values later.