non-zero threshold for event function

I have written the following event function. Since, I have to detect a non-zero event, I have used Boolean operator as shown.

function [value,isterminal,direction] = myevent12d1(t,y)
    % Locate the time when height passes through zero in a decreasing direction
    % and stop integration.

    z=y(1)<y(3)
    value = z;     % detect height = 0
    isterminal = 1;   % stop the integration
    direction = -1;   % negative direction
    
    

However, when I run my code, I recieve the following error

Undefined function 'sign' for input arguments of type 'logical'.

Error in odezero (line 46)
    indzc = find((sign(vL) ~= sign(vR)) & (direction .* (vR - vL) >= 0));

Error in ode45 (line 352)
    [te,ye,ie,valt,stop] = ...

Error in onedof2dof (line 19)
            [t1,y1,te1,ye1,ie1]=ode45(@(t,y) sp121(t,y,k1,k2,m1,m2),[tstart tfinal],y0,options1);

            

options1 is defined as:

options1 = odeset('Events',@myevent12d1);

Please help me in this reagrd. Thanks.