Trying to use patch to fill between two lines but no shading due to empty double row vector?

I am trying to shade between z and v up to 0.725 but it just returns

idx =

1×0 empty double row vector

How do I get this to shade the space?

clear all

close all

clc

% V ideal

y = 0:10:35000;

x = 0:.01:0.9;

y = 14490 .* log( 1 ./ (1 - x));

% n max

z = 0:1:35000;

b = 0:0.01:0.9;

z = 14490 .* (log( 1 ./ (1 - b)) - ( 0.25 ) .* ( b ./ (1-b)));

% n = 1.1

v = 0:1:35000;

b = 0:0.01:0.9;

v = 14490 .* (log( 1 ./ (1 - b)) - (( 0.9091 ) .* b));

hold on

% All Functions Plotted

plot(x,y,'g');

plot(b,z,'r');

plot(b,v);

% Fill Area

idx = find(b==0.725)

patch([b(1:idx) fliplr(b(1:idx))],[z(1:idx) fliplr(v(1:idx))],'b')

% Labeling the Plot

title('\DeltaV vs. m_p/m_0')

xlabel('m_p/m_0')

ylabel('\DeltaV')

label1 = '\DeltaV Ideal';

label2 = '\eta_{max} = 4';

label3 = '\eta = 1.1';

legend(label1,label2,label3,'interpreter','tex')

hold off

Answer 1

[DGM]

That's because there is no x-value at that location due to its step size. See the comment on the prior question.