how to summarize a datatable in workflow (i get an error with this syntax: dataTable1.Compute("Sum(Acres)", "STATE_ABBR = 'OR'")

I am getting an error with this syntax in the workflow designer.

 

what i want to do is summarize an acres field in a datatable for a particular value.

 

dataTable1.Compute("Sum(Acres)", "STATE_ABBR = 'OR'")

 

the syntax does not give an error in the intellasense in the workflow designer.  any suggestions on how to summarize data in a datatable within workflow.

 

Jeff