I suspect this might be related to the problem I had with using kinds as a column in a table (Testing if an object is a certain type - using tables). Basically sai is trying to switch on a kind just like I was trying to compare an object to a kind from a table.
I suspect you could solve it using the same solution you suggested for me. In other words, add some property to each body part (with a different value for each kind) that you can match on instead.