what's wrong with the Matlab code below? How to fix it?

a1=3;

b1=5;

k=0;

s=(a1+b1)/2;

while abs(bessely(0,s))>0.01

if bessely(0,s)>0

b1=s;

else

a1=s;

end

s=(a1+b1)/2;

k=k+1;

end