Think of what the average and covariance mean in this case. If you only have a single sample, then:
the average is your only sample
there is no sample at a non-zero distance from the average, hence the covariance is zero.
Edit Note that if you wanted to calculate the average and variance of the 390 float values, you need to use CV_COVAR_COLUMNS
instead of CV_COVAR_ROWS
.