'Simple' SQL Problem selecting null dates

Hi,
This really should be simple, but I'm having strange behaviour on the following code:
This works fine (returns 31 rows):
DECLARE @numInstances int
EXEC      @numInstances = sp_executesql N'SELECT     COUNT(*) AS numInstances
FROM         xjrvw_TR_Instances
WHERE     (fkResponsiblePersonId = @ResponsiblePersonId) AND (fkOrganizationId = @OrganizationId) AND (TraceDate IS
NULL)',N'@ResponsiblePersonId int,@OrganizationId int',@ResponsiblePersonId=1,@OrganizationId=2

This returns 0 rows:
DECLARE @numInstances int
EXEC      @numInstances = sp_executesql N'SELECT     COUNT(*) AS numInstances
FROM         xjrvw_TR_Instances
WHERE     (fkResponsiblePersonId = @ResponsiblePersonId) AND (fkOrganizationId = @OrganizationId) AND (TraceDate IS
NULL)',N'@ResponsiblePersonId int,@OrganizationId int',@ResponsiblePersonId=1,@OrganizationId=2
GO
And this also returns 0 rows (from the SQL Profiler when I run through the app)
DECLARE @numInstances int
EXEC      @numInstances = sp_executesql N'SELECT     COUNT(*) AS numInstances
FROM         xjrvw_TR_Instances
WHERE     (fkResponsiblePersonId = @ResponsiblePersonId) AND (fkOrganizationId = @OrganizationId) AND (TraceDate IS
NULL)',N'@ResponsiblePersonId int,@OrganizationId int',@ResponsiblePersonId=1,@OrganizationId=2
GO

I have tried changing it to a store procedure, to no avail.  The problem is definitely with the TraceDate IS NULL  - it works without that part of the predicate. I tried TraceDate.ISNULL = 1 but it tells me 'Cannot call methods on datetime.'

Can anyone please help?  I need to run it through the app, which interprets the top code to the second version.  

Thanks!
Carol.

exec sp_executesql N'SELECT     COUNT(*) AS numInstances
FROM         xjrvw_TR_Instances
WHERE     (fkResponsiblePersonId = @ResponsiblePersonId) AND (fkOrganizationId = @OrganizationId) AND (TraceDate IS 
NULL)',N'@ResponsiblePersonId int,@OrganizationId int',@ResponsiblePersonId=1,@OrganizationId=2
GO
DECLARE @numInstances int
EXEC	@numInstances = sp_executesql N'SELECT     COUNT(*) AS numInstances
FROM         xjrvw_TR_Instances
WHERE     (fkResponsiblePersonId = @ResponsiblePersonId) AND (fkOrganizationId = @OrganizationId) AND (TraceDate IS 
NULL)',N'@ResponsiblePersonId int,@OrganizationId int',@ResponsiblePersonId=1,@OrganizationId=2
GO
DECLARE @ResponsiblePersonId int
SET @ResponsiblePersonId = 2
DECLARE @OrganizationId int
SET @OrganizationId=1
SELECT     COUNT(*) AS numInstances
FROM         xjrvw_TR_Instances
WHERE     (fkResponsiblePersonId = @ResponsiblePersonId) AND (fkOrganizationId = @OrganizationId) AND (TraceDate IS 
NULL)

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