扩展例外：乌龟时输入列表为空错误... [netlogo]

Question

在运行以下代码时

 set m2 []

ask turtles [
  set m2 matrix:from-column-list map [ i -> [ ownlist2] of i ] sort 
  flockmates
 ]
 ;where flockmates refer to agentset of neighbouring turtles

有一个运行时错误说：扩展例外：乌龟netlogo时输入列表为空错误，...