Select CASE returning NULL value

I have the following CASE statement in a view:

(SELECT CASE
WHEN ISNULL([dbo].[tblContact].[JobTitleID], NULL) = NULL
THEN [dbo].[f_ContactNameFL]([dbo].[tblContact].[ContactID])
ELSE [dbo].[f_ContactNameFL]([dbo].[tblContact].[ContactID]) + ', ' +
[dbo].[tlkpJobTitle].[Title]
END) AS ContactNameAndTitle

It works perfectly when dbo.tblContact.JobTitleID is NOT null. When it
is null it should return a contact name but it is returning null. The
problem is not f_ContactNameFL. There is another column in the same
view that uses that function and it works perfectly. What am I doing
wrong?

Thanks,
Shelley