Power BI Dataflow "Param1 = Value cannot be null"

I am trying to create a Power BI dataflow using the Azure Analysis Services connector for the first time. Everything looks good when I'm building the dataflow, but I am unable to refresh at all. When I hit refresh it processes for about a second and returns an error - "Error: An internal error occurred.. Param1 = Value cannot be null."

I've tried removing blanks and nulls from the table, connecting to different tables in the same model, and deleting and recreating the dataflow, but nothing has any impact.

Has anyone experienced this error or have any ideas on what I could try to get the dataflow to refresh? I would really appreciate any thoughts or suggestions. Thank you!

Advanced Editor:

let
  Source = AnalysisServices.Databases("powerbi://api.powerbi.com/v1.0/myorg/procurement%20analyst%20reports"),
  #"Navigation 1" = Source{[Name = "Products and Orders"]}[Data],
  #"Navigation 2" = #"Navigation 1"{[Id = "Model"]}[Data],
  Model = #"Navigation 2"{[Id = "Model"]}[Data],
  #"Added items" = Cube.Transform(Model, {{Cube.AddAndExpandDimensionColumn, "[FactVendorPurchaseOrderItem]", {"[FactVendorPurchaseOrderItem].[ETA].[ETA]", "[FactVendorPurchaseOrderItem].[PurchaseOrderNumber].[PurchaseOrderNumber]"}, {"ETA", "PurchaseOrderNumber"}}}),
  #"Filtered rows" = Table.SelectRows(#"Added items", each ([ETA] <> null) and ([PurchaseOrderNumber] <> null and [PurchaseOrderNumber] <> ""))
in
  #"Filtered rows"