MDX query returns error

This MDX returns data:

Select  [Accounting Date].[Year Month].members ON rows,
[Measures].[Chart of Account Balance - Actual]  on columns
 from [Financials]

And this MDX returns data:
 Select  [Financial Statement].[GL Number].members on rows,
 [Measures].[Chart of Account Balance - Actual]  on columns
 from [Financials]


But this MDX returns the error:

Two sets specified in the  function have different dimensionality.

Select  [Accounting Date].[Year Month].members ON columns,
{[Financial Statement].[GL Number].members, [Measures].[Chart of Account Balance - Actual]}  on rows
 from [Financials]

 And... If I remove the braces I get the error:

The statement dialect could not be resolved due to ambiguity.

What am I doing wrong? Thanks!