Kangaroo - constrain to surface error

I'm working on a sort of bubble packing along a surface and it appears to be pretty simple using Kangaroo. I'm using springs with a constrain to surface. The springs work just fine, but the constrain to surface gives the error:

"1. Solution exception:Index was out of range. Must be non-negative and less than the size of the collection. Parameter name: index."

This happens if I plug it into force object by itself or with springs and messing around with the list structure doesn't change the result. Any ideas where I'm going wrong? The GH file is attached with Rhino geometry internalized.

Thanks,

Alex