Calculated Measure

I have a calculated measure with the mdx expression like:

sum(
{
[Plano Contas Geral].[Parent].&[23.1.2],
[Plano Contas Geral].[Parent].&[22.1],
[Plano Contas Geral].[Parent].&[22.2],
[Plano Contas Geral].[Parent].&[22.8],
[Plano Contas Geral].[Parent].&[26.9],
[Plano Contas Geral].[Parent].&[21.9],
[Plano Contas Geral].[Parent].&[26.1.8]
}, [Measures].[Credito]
)

But, sometimes there some members in the set specified in the sum function that do not exist, in this case I get an error when
olap engine parses it. How can I get it to work, even when one of the members doesn't exist?

Help would be appreciated,

Luis Silva