Analytic function for conditional reset ROW_NUMBER when a second column has a NULL value

Hello All,
I have in TeraData :

Select PER_ID, CONTACT_ID, START_DT, PREV_CONTACT_ID,
ROW_NUMBER() Over (Partition By PER_ID Order By START_DT RESET WHEN PREV_CONTACT_ID Is Null) As CONTACT_SERIES
 
Explanation: if in partition a column PREV_CONTACT_ID has a value NULL, in this case the ROW_NUMBER must restarts from 1.

But in Vertica does not exists: 'OVER (.... RESET WHEN condition)'. How can I write similar condition in Vertica?

Thanks,
Heiki