"Opaque errors" happens a lot. The thing I've found with the documentation is that quite often, it will say exactly what it needs, but the wording is not perhaps as clear as it could be. This kind of enigma wrapped up in a conundrum can occur when the documentation writer knows the subject so well that it is (unfortunately) assumed that the reader has the same level of understanding.
What the documentation needs is a complete overhaul with working examples provided for ALL of the available functions. Take for instance an issue I'm currently working on regarding Content Centre members:
For reasons known only to the developers themselves, it appears as though ContentTableRows and ContentFamilyIds have a unique {guid} identifier that's visible when you edit a Content Center table in Excel (unhide columns A & B and scroll down) but to Inventor, there's also an added string element that wraps around that same string value. In the case of ContentFamily it's this kind of thing: "v3#" + 5856551f-561e-4115-bc3f-0544a13a4940 + "#" whereas for the ContentTableRow it takes this format: "v3#" + 5856551f-561e-4115-bc3f-0544a13a4940 + "#10" - puzzling to say the least!
EDIT: So after stepping through a couple of rows in the ContentTable in question, the second "#string" part actually makes sense- my point about why did family have "v3#" at the beginning stands however- unless it's a versioning thing(?) which seems like a terrible idea, because anyone that hard-codes something to look for this is going to have a bad time if the version ever changes!?