private var cellCoords = new List.<Vector3>();
private var occupiedCoords = new List.<Vector3>();

function randomUnoccupiedCoord () {
    var unoccupiedCells = new List.<Vector3>();
    unoccupiedCells = cellCoords.Except( occupiedCoords ).ToList(); // Line 46, the error line
    return unoccupiedCells;
}

// The Error
// InvalidCastException: Cannot cast from source type to destination type.
// gameDriver.randomUnoccupiedCoord () (at Assets/Rewrite/gameDriver.js:46)
// gameDriver.Start () (at Assets/Rewrite/gameDriver.js:21)