Problem in setting Null to the Foreign key reference .

I am having a table.

When i delete a record in that table, the foreign key refences table should set to Null.

I tried the following code but it is not working fine.

ALTER TABLE E_InvoiceMeterReadings

ADD CONSTRAINT [FK_E_InvoiceMeterReadings_E_InvoiceTmp] FOREIGN KEY([InvoiceTmpID])

REFERENCES [dbo].[E_InvoiceTmp] ([InvoiceTmpID])

ON DELETE SET NULL

can any one help me in this issue.