Should have just given the formula for center of mass,

mass1position1 + mass2position2 ... / mass1 + mass2 ...

or at least a better hint

null does not have any methods, in particular it does not have a filter() method, this is why your code throws an error when called with null. this is consistent across all JavaScript implementations and has nothing to do with the environment. you have to add an explicit guard for null, since it is a valid input in this kata (which is not good practice, but that's besides the point)