IllegalArgumentException: name 'x' not in index

[giottolee]

i'm tring to use dataframe calculating the statistic feature, i don't know why it throws IllegaArgumentException. I debug it and it looks right. This is my code and dataframe object.

String dimension = "x";
Double mean = (Double) sensorSlice.mean().col(dimension).get(0);
Double max = (Double) sensorSlice.max().col(dimension).get(0);
Double min = (Double) sensorSlice.min().col(dimension).get(0);
Double std = (Double) sensorSlice.stddev().col(dimension).get(0);