It means it tried to invert a matrix and got something surprisingly noninvertible. Basically, I think it means you’ve got nonsense in the matrix you tried to invert.
It’s not something simple like a scale to zero (which is the way that most noninvertible matrices are accidentally constructed), because a scale to zero would have been detected earlier in the invert process. So it must be some other problem with the matrix that makes it noninvertible, and since I’ve never run into this assertion before, I suspect it’s something pretty unusual with the matrix.
Maybe the matrix is just incorrectly transposed or something like that.
David