Update AD values

Hello,

We are trying to loop through a recordset to update AD values that have changed.  If the value for a particular property in AD already exists (ie middle initial) then updating that value to another works great.  The problem arrises that when the value has not been set and we try to update it the following fails:

If Not Trim(rs.Fields(f)) = objUser.get(f) Then

The error is:

The directory property cannot be found in the cache.

I understand why this is happening but not sure how to check for the issue and insert the new value.I update the values lie this:

objUser.Put f, Trim(rs.Fields(f))

Any ideas would be great.

Ernst.