Visual Studio file input NullReferenceException was unhandled c#

Question

I have tried looking at other peoples questions to find a solution however just cannot understand what/why this is happening. If someone can take a look at this code and explain why this is causing it may give me a better understanding

private animalClass[] fileToAnimalArray(String fileName)
    {
        // read the number of lines in the file and declare animal array with exact number needed
        int lineCount = File.ReadLines(fileName).Count();
        animalClass[] animalArray = new animalClass[lineCount];

        //declare temp variables to hold a single read out from the file and a string array to hold the split up string
        String line;
        String[] lineArray = new String[10];

        // open up file in prepartion to read file
        StreamReader file = new StreamReader(fileName);

        // start loop in order to extract each line of the file
        for (int i = 0; i < lineCount; i++)
        {
            // read line and spllit it
            line = file.ReadLine();
            lineArray = line.Split(',');
            string temp = lineArray[0];
            // assign split string into each property of the animal array casting if needed.
            animalArray[i].aClass = lineArray[0];  // *error thrown here*
            animalArray[i].aOrder = lineArray[1];
            animalArray[i].aFamily = lineArray[2];
            animalArray[i].aGenus = lineArray[3];
            animalArray[i].aName = lineArray[4];
            animalArray[i].aLength = Convert.ToDouble(lineArray[5]);
            animalArray[i].aWeight = Convert.ToDouble(lineArray[6]);
            animalArray[i].extState = lineArray[7];
            animalArray[i].recordCreationDate = Convert.ToDateTime(lineArray[8]);
            animalArray[i].recordLastEdit = Convert.ToDateTime(lineArray[9]);

        }

        // close the file
        file.Close();

        // return the array
        return animalArray;
    }

many thanks, Cameron