SSAS Calculated Measure Error

I have a calculated measure, below, which is coming back with an error.

"({[WFM Exceptions].[Actual Exception].children-[WFM Exceptions].[Actual Exception].&[Intermediate Work]},[Measures].[TimePerUnit])"

Function Expects a string or numeric expression. A tuple set was used.

Any idea's? I have used the above, and a variant with the EXCEPT function and both return the same issue.