CDC function should generate an error, but doesn't

When I execute this following code, the null as the second parameter to the get_all_changes function should generate an error back to the user.  However, it doesn't.  It just returns an empty result set.  If I don't have a where clause, it does generate the expected error.  Does anyone know why this behaves in this manner?

SELECT  *
FROM cdc.fn_cdc_get_all_changes_dbo_Table1(0x01, null, 'all')
WHERE
CAST(sys.fn_cdc_is_bit_set(1, __$update_mask) AS int) >0

Thanks,

Jason

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