Axios call, argument must be array but null given

In my vue component I have a data array

data: {
  categories: [{id:1, title: "category 1"},{id:2, title: "category 2"}]
}

and I’m trying to load this in as a data argument in an axios call

axios({
  method: 'get',
  url: //testURL,
  data: {
    categoryCall: this.categories
  }
}).then(function (response){
  console.log(response);
}).catch(function (error){
  console.log(error.response);
});

but I’m getting a 400 bad request code where it tells me that argument 1 should be an array and null is given.

Am I not properly passing in the already established array in the axios call? Where am I going wrong?