I figured out what was going on. It was complicated.
I was creating a matrix starting with
Value::Matrix, and using Davide Cervone's
assignmacro to modify the entries. This is a BAD THING; the modified matrix was causing the error message. When I created the matrix using other methods (setting up an array), the error message went away.
Once again, I'm posting my reply to help those who may find themselves in another situation.