function defuzz with the 'mom' option

This is a code using a function 'defuzz' in the fuzzy logic toolbox. It gives a answer 0 -0.5 0. I doubt that the result(0) of 'som' is right.

x= -1:0.5:1; 
mf= [0.1 0.8 0.8 0.8 0.1];
a1= defuzz(x,mf,'som'); a2= defuzz(x,mf,'lom'); a3= defuzz(x,mf,'mom');
[a1 a2 a3]