SQL Server - Case Statement resulting in NULL

I have a result set in my query that contains empty strings like ' '.  I would like to make those NULL in the select clause in my sql statement like this...

CASE WHEN Value = ' ' THEN NULL END AS Value

But when I do this I get this error...

None of the result expressions in a CASE specification can be NULL.

Is there a better way to accomplish what I'm after?

Thanks