Invalid Object Name Error – Scalar Function

I have a scalar valued function that uses the following parameters:
@birthmonthnum,@birthdaynum and @birthyearnum

I can run the function if I use:
select dbo.fnc_GetBirthDate (6,21,1959)as birthdate

But if I try to join with another table using:
SELECT * FROM cv3client c CROSS APPLY dbo.fnc_GetBirthDate (c.birthmonthnum,c.birthdaynum,c.birthyearnum)
I get the following error:
Invalid object name ‘dbo.fnc_GetBirthDate’.

Any help would be appreciated.