SQL Server 2005: column cannot be persisted because is non-deterministic

Question

I have a table with a calculated field (Test) based on the result of a function (MyFunctionTest) and i want make it PERSISTED and add an index for performance improvments, eg.:

CREATE TABLE [dbo].[TestTable] (
    [Id] [int] IDENTITY(1,1) NOT NULL,
    [Foo] [int] NOT NULL,
    [Test]  AS ([dbo].[MyFunctionTest]([Foo])) PERSISTED,
    CONSTRAINT [PK_TestTable] PRIMARY KEY CLUSTERED 
(
    [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] TEXTIMAGE_ON [PRIMARY]

MyFunctionTest is:

ALTER FUNCTION [dbo].[MyFunctionTest]
(
    @foo int
)
RETURNS int
AS
BEGIN
    RETURN @foo * 2
END

But, sql server 2005 on save show the error

Computed column 'Test' in table 'Tmp_TestTable' cannot be persisted because the column is non-deterministic.

why MyFunctionTest isn't deterministic?

SOLUTION

Add WITH SCHEMABINDING to the function make it deterministic

ALTER FUNCTION [dbo].[MyFunctionTest]
(
    @foo int
)
RETURNS int
WITH SCHEMABINDING
AS
BEGIN
    RETURN @foo * 2
END