locust recording trail error

I am trying to show the trail of a swarm using locust,but I am having trouble as there is an error in the script. when I run the script 'prieview' goes red and the error is 'data conversion failed from integer to geometry' This is then effecting the gradient (I want the older part of the trail to be a lighter colour/fade in the distance).

I am also using millipede, and weaverbird.