Data Validator Read Allowed Values from Another Step "Read allowed value"

I'm trying to use the data validator step to look for bad/missing data. It does fine at checking for nulls, data types, max, min, and values that match an explicitly entered list. What doesn't work is the "Read Allowed Values from another step". If I try to use this the transform fails as soon as it gets to the data validator. I get several different error messages but commonly it says that it's unable to find the variable that it's supposed to be checking. I've been playing with this for a couple of days without joy. My external step is just a datagrid with one field called apgar and permitted values of 0,1,..10, 99. When I specify this as an external step it sees it fine and recognises the field. I've tried do a validation without the data grid and without checking other fields and everything is fine but as soon as I add the data grid step and connect it to the data validator the transform fails, even though at that point I haven't added the validation check that references it. My hunch is that it's getting confused about where it's supposed to look for it's input.

I've searched all over the forum and the web and I only found one reference to something similar. That was from 2010 and the post said that this feature simply doesn't work.

Any ideas anyone?

Thanks
Nick