How do I prevent errors with empty string date fields passed with CFSTOREDPROC to Sql Stored Procedure?

Front-end webpage:
 
<cfif parameterexists(updated)>
<cfstoredproc procedure="client_update" datasource="#dsn#">
<cfprocparam variable="@clientid" value="#id#" cfsqltype="cf_sql_smallint">
<cfprocparam variable="@firstname" value="#firstname#" cfsqltype="cf_sql_varchar">
<cfprocparam variable="@lastname" value="#lastname#" cfsqltype="cf_sql_varchar">
<cfprocparam variable="@phone" value="#phone#" cfsqltype="cf_sql_varchar">
<cfprocparam variable="@phone2" value="#phone2#" cfsqltype="cf_sql_varchar">
<cfprocparam variable="@email" value="#email#" cfsqltype="cf_sql_varchar">
<cfprocparam variable="@type" value="#type#" cfsqltype="cf_sql_varchar">
<cfprocparam variable="@listdate" value="#listdate#" cfsqltype="cf_sql_date">
<cfprocparam variable="@tailnumber" value="#tailnumber#" cfsqltype="cf_sql_varchar">
<cfprocparam variable="@listprice" value="#listprice#" cfsqltype="cf_sql_varchar">
<cfprocparam variable="@location" value="#location#" cfsqltype="cf_sql_varchar">
<cfprocparam variable="@website" value="#website#" cfsqltype="cf_sql_varchar">
<cfprocparam variable="@contacted" value="#contacted#" cfsqltype="cf_sql_varchar">
<cfprocparam variable="@lastcontactdate" value="#lastcontactdate#" cfsqltype="cf_sql_date">
<cfprocparam variable="@initials" value="#initials#" cfsqltype="cf_sql_varchar">
<cfprocparam variable="@status" value="#status#" cfsqltype="cf_sql_varchar">
<cfprocparam variable="@followupdate" value="#followupdate#" cfsqltype="cf_sql_date">
<cfprocparam variable="@archive" value="#archive#" cfsqltype="cf_sql_varchar">
<cfprocparam variable="@comments" value="#comments#" cfsqltype="cf_sql_varchar">
 
<cfprocresult name="clientupdate">
</cfstoredproc>
 
Stored Procedure:
****** Object:  StoredProcedure [dbo].[client_update]    Script Date: 01/04/2008 21:36:45 ******/
@clientid smallint,
@firstname varchar (50),
@lastname varchar (50),
@phone varchar (25),
@phone2 varchar (25),
@email varchar (50),
@type varchar (50),
@listdate smalldatetime,
@tailnumber varchar (10),
@listprice varchar (12),
@location varchar (50),
@website varchar (250),
@contacted varchar (10),
@lastcontactdate smalldatetime,
@initials varchar (2),
@status varchar (10),
@followupdate smalldatetime,
@archive varchar (3),
@comments varchar (1000)
 
AS
 
UPDATE client
SET
 
firstname=@firstname,
lastname=@lastname,
phone=@phone,
phone2=@phone2,
email=@email,
type=@type,
listdate=@listdate,
tailnumber=@tailnumber,
listprice=@listprice,
location=@location,
website=@website,
contacted=@contacted,
lastcontactdate=@lastcontactdate,
initials=@initials,
status=@status,
followupdate=@followupdate,
archive=@archive,
comments=@comments
 
WHERE clientid=@clientid
 
GO