MATLAB: Running code and getting error about vector must be the same length

I'm running my code and I am getting an error about how the vector must be the same length. Does anyone have advice because I thought the vectors were fillled to the same length?

ntrails = 20;
N_run = zeros(ntrails,1);
BC = [5,-5,-5,5];
for trail = 1:1:ntrails
    xA = -5; xAK = xA;
    yA= 0; yAK = yA;
    xB = 5; xBK = xB;
    yB = 0; yBK = yB;
    nsteps = 1000;
    collision_flag = 0; k = 0;
 while collision_flag == 0 && k < nsteps
     [xAKp1, yAKP1] = RandWalk_2D(xAK,yAK,BC);
     [xBKp1, yBKP1] = RandWalk_2D(xBK,yBK,BC);
     
     xAK_val = [xAK-.5, xAK + .5, xAK + .5, xAK -.5];
     yAK_val = [yAK-.5, yAK + .5, yAK + .5, yAK - .5];
     
     xAKp1_val = [xAKp1-.5, xAKp1 + .5, xAKp1 + .5, xAKp1 -.5];
     yAKp1_val = [yAKp1-.5, yAKp1 + .5, yAKp1 + .5, yAKp1 - .5];
          
     xBK_val = [xBK-.5, xBK + .5, xBK + .5, xBK -.5];
     yBK_val = [yBK-.5, yBK + .5, yBK + .5, yBK - .5];
     
     xBKp1_val = [xBKp1-.5, xBKp1 + .5, xBKp1 + .5, xBKp1 -.5];
     yBKp1_val = [yBKp1-.5, yBKp1 + .5, yBKp1 + .5, yBKp1 - .5];
     
     figure(1)
     hold on
     xlim([-5.5,5.5,])
     ylim([-5.5,5.5,])
     fill(xAK_val,yAK_val,'r') %current step A
     fill(xAKp1_val, yAKp1_val, 'b')%next step for A
     fill(xBK_val,yBK_val,'r') %current step B
     fill(xBKp1_val, yBKp1_val, 'b')%next step for B
     hold off
     
     xAK = xAKp1; yAK = yAKp1; 
     xBK = xBKP1; yBK = yBKp1;
     k = k+1;
     
    if xAK == xBK && yAK == yBK
        collision_flag = 1;
        N_run(trail) = k;
    end
 end
end
 medval = median(N_run)
    
function [x,y] = RandWalk_2D(x0,y0,BC)
   r = rand;
   if r<0.2
        x = x0;
        y = y0+1;
        if y >= BC(1); y = BC(1);
        end
   else if 0.2 < r && r <= 0.4;
           x = x0;
           y = y0-1;
           if y <= BC(2);
               y = BC(2);
           end
       elseif 0.4< r && r<=0.6
           x = x0 - 1;
           y = y0;
           if x<= BC(3); x = BC(3)
           end
       elseif 0.6 < r && r <= 0.8;
           x = x0+1; y = y0;
           if x >= BC(4); x = BC(4);
           end
       elseif 0.8<r 
           x = x0; y = y0;
       end
   end
end

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