Only way I can replicate this is if cells that the arguments refer to
are returning #N/A.
Otherwise the errors returned are usually #NAME? (Analysis ToolPak not
installed or selected as an Addin or reference to a non-existent
name), and #NUM! (invalid data inputs). See Help on both of those.
I had this when I was using the drop-down wizard thing to fill it in, but
didn't notice the scroll bar and therefore didn't see the prompt for the
final parameter. Ensure you've *really* got it all filled out -- the last
item is "Type" not "End_Period"