Vue CLI + axios + slice problems

Hello! I’m currently working on a project, with vue cli and axios. I’m having troubles with my code, I have a list from a api (petrol stations in iceland) and I only want to show the top 3, or the cheapest ones. One list with only petrol, the other with diesel. In the browser I get the following errors:

1: [Vue warn]: Error in render: “TypeError: _vm.stationsPetrol.slice is not a function”
2: TypeError: _vm.stationsPetrol.slice is not a function.

Here is my code:
Can anyone tell me what I’m doing wrong?