i have a ,MAV problem.

i wrote this code while performing features extraction for MAV using convolution

MAV1 = zeros(length(yt1), 1);

var1 = zeros(length(yt1), 1);

MAV1 = conv(abs(real(yt1)), filter, 'same');

i get an error on MAV1 in the first part of the code, why so? any assistances