You cannot skip positional parameters. It should be

...
EXEC bm_GetClients NULL, @ClientKey
--or
...
SET  @CompanyKey = NULL
EXEC bm_GetClients @CompanyKey, @ClientKey

Named parameters will work as well

EXEC bm_GetClients @ClientKey =  @ClientKey

>Ok, here it is.
>

>
>SET ANSI_NULLS ON
>GO
>SET QUOTED_IDENTIFIER ON
>GO
>
>CREATE PROCEDURE bm_GetClients
>	@CompanyKey		INT = NULL,
>	@ClientKey		INT = NULL
>
>AS
>BEGIN
>
>	SELECT *
>		FROM bm_Clients
>		WHERE (CompanyKey = @CompanyKey OR @CompanyKey IS NULL) AND
>			  (ClientKey = @ClientKey OR @ClientKey IS NULL)
>END
>

>
>If I call it with:
>

>DECLARE @CompanyKey INT
>DECLARE @ClientKey INT
>SET  @CompanyKey = 2
>SET @ClientKey = 5
>EXEC  bm_GetClients @CompanyKey = @CompanyKey, @ClientKey = @ClientKey
>

>
>Then it works ok.
>
>