Why I get the eigen value as $0$ for the minimum variance?

Question

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like equation (8.8) for $M_{ii}^B = <B_i B_i> - <B_i><B_i>$ is coming $0$ for my time series data set. But this eigen value should not be $0$ as described in the book. What I am doing wrong! Thanks!

• Here < > refers the mean and I have used Matlab for the data set