问在SQLServer 2005上的INSERT INSERT INSERT(*)=0的唯一键约束？

System.Data.SqlClient.SqlException: Violation of UNIQUE KEY constraint 'UK1_MyTable'. Cannot insert duplicate key in object 'dbo.MyTable'.
The statement has been terminated.

INSERT INTO MyTable (FieldA, FieldB, FieldC)
SELECT FieldA='AValue', FieldB='BValue', FieldC='CValue'
WHERE (SELECT COUNT(*) FROM MyTable WHERE FieldA='AValue' AND FieldB='BValue' AND FieldC='CValue' ) = 0

CREATE TABLE [dbo].[MyTable](
  [Id] [bigint] IDENTITY(1,1) NOT NULL,
  [FieldA] [bigint] NOT NULL,
  [FieldB] [int] NOT NULL,
  [FieldC] [char](3) NULL,
  [FieldD] [float] NULL,
  CONSTRAINT [PK_MyTable] PRIMARY KEY NONCLUSTERED 
  (
    [Id] ASC
  )WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON),
  CONSTRAINT [UK1_MyTable] UNIQUE NONCLUSTERED 
  (
    [FieldA] ASC,
    [FieldB] ASC,
    [FieldC] ASC
  )WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON)
)