Whereclause for process step

Hi,

I have a process that looks for stale accounts.  We import last email sent and last logon into fields in the person record and then deactivate the account if it's been stale for 90 days.  The following where clause is in the step:

f.Comparison("lastreallogon", Date.Today.AddDays(-90), ValType.Date, CompareOperator.LowerThan, FormatterOptions.None), _

                                                  f.Comparison("lastemailsent", Date.Today.AddDays(-90), ValType.Date, CompareOperator.LowerThan, FormatterOptions.None), _

                                                  f.Comparison("XDateInserted", Date.Today.AddDays(-30), ValType.Date, CompareOperator.LowerThan, FormatterOptions.None), _

Now.... This works fine for a person that has logged on, but is stale.  If either lastreallogon or lastemailsent have a null value, it doesn't evaluate in the whereclause and the process step doesn't execute.  What is the right syntax for also checking for Null values and having a Comparison for isNull?

Thanks