Why do I get this error message: incomparable types: double and nulltype

Hello,

I am building a function to interpolate eight lists (it's a 3D interpolation with lists considered as edges of a cube).

The lists arrive in different inputs and are put in a 2D array. As the lists might have different lengths and the process begins as soon as a list is received, I first seek the length of longest list. Then prior to interpolate I want to check if the eight values do exist (non-null) otherwise I use zeroes. However when I compile I get an error message for this line

if (items[j][i] == null) {

The error message is: incomparable types: double and
if (items[j][i] == null) {

The complete function is:

//interpolation
    public static double interpolate(int numItems, double[] position, double[][] items) {
        double[] interpolatedValues = new double[numItems];
        for (int i=0; i

I don't understand why I get this error message.