how to fix 'Cannot read property of undefined'?

I'm trying to determine which object in an array has the longest name and logging that object to the console.

I can't seem to access the length of the name property in my if statement.

  let longest;
  for (let i = 0; i < instructors.length; i++) {
    if (instructors[i].name.length > longest.length) {
      longest = instructors[i];
    }
  }
  return longest;
};

console.log(instructorWithLongestName([
  {name: "Samuel", course: "iOS"},
  {name: "Jeremiah", course: "Web"},
  {name: "Ophilia", course: "Web"},
  {name: "Donald", course: "Web"}
]));
console.log(instructorWithLongestName([
  {name: "Matthew", course: "Web"},
  {name: "David", course: "iOS"},
  {name: "Domascus", course: "Web"}
]));

I expect the output of
{name: "Jeremiah", course: "Web"}
{name: "Domascus", course: "Web"}

but I get an error stating it cannot read property '.length'