Unique Index

I am trying to create an unique index on the following table that stores relationship information between two persons with constraints where [code="plain"][size="1"][end_date] IS NULL OR [end_date] >= GETDATE()[/size][/code][code="plain"][size="1"]CREATE TABLE [dbo].[person_relationship]( [person_relationship_id] [bigint] IDENTITY(1,1) NOT NULL, [person_id_1] [bigint] NOT NULL, [person_id_2] [bigint] NOT NULL, [person_relationship_type_id] [bigint] NOT NULL, [start_date] [datetime] NOT NULL, [end_date] [datetime] NULL, CONSTRAINT [PK_person_relationship] PRIMARY KEY CLUSTERED ( [person_relationship_id] ASC)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]) ON [PRIMARY]ALTER TABLE [dbo].[person_relationship] WITH CHECK ADD CONSTRAINT [FK_person_relationship_1_person] FOREIGN KEY([person_id_1])REFERENCES [dbo].[person] ([person_id])GOALTER TABLE [dbo].[person_relationship] CHECK CONSTRAINT [FK_person_relationship_1_person]GOALTER TABLE [dbo].[person_relationship] WITH CHECK ADD CONSTRAINT [FK_person_relationship_2_person] FOREIGN KEY([person_id_2])REFERENCES [dbo].[person] ([person_id])GOALTER TABLE [dbo].[person_relationship] CHECK CONSTRAINT [FK_person_relationship_2_person]GO[/size][/code]The following is the unique index I am attempting to create but SQL is not having it.[code="plain"][size="1"]CREATE UNIQUE INDEX [IX_person_1_person_2_relationship_type]ON [dbo].[person_relationship]([person_id_1], [person_id_2], [person_relationship_type_id])WHERE ([end_date] IS NULL) OR ([end_date] >= GETDATE())[/size][/code]The issues are "OR" and "GETDATE()". I found out that using date functions result in error but not sure why "OR" is causing grief. Any advise/suggestions will be deeply appreciated.