Assuming you are using the usual definition of covariance
then it should be undefined. You will end up evaluating things like and because your first array is apparently 1 x 5 and your second array is 5 x 1 this is undefined (when you explicitly spelled out the dimensions this is what you get). If you got it backwards, it is still undefined for the same reasons. You need xy' to be defined to get an answer. Normally it is a 5 x 5 matrix in this case if they are both column vectors, or a scalar if they are both row vectors.