Fluctuating and negative value in pdepe

I am trying to solve a coupled pde in MATLAB. The code is :

function pdex5
m = 0;
x = linspace(0,1,100);
t = linspace(0,6,400);
options=odeset('Reltol',1e-10,'Abstol',1e-8);
sol = pdepe(m,@pdex5pde,@pdex5ic,@pdex5bc,x,t,options);

u1 = sol(:,:,1);
u2 = sol(:,:,2);

figure
plot(x,u1(end,:))
title('Solution at t = 5')
xlabel('Distance x')
ylabel('u1(x,5)')

figure
plot(x,u2(end,:))
title('Solution at t = 5')
xlabel('Distance x')
ylabel('u2(x,5)')

% --------------------------------------------------------------
function [c,f,s] = pdex5pde(x,t,u,DuDx)
De=.00005; strainrate=0.2;
c = [De; 1]; 
f = [0;(1+(u(1))^1.2)] .* DuDx; 
s = [1;0]+flipud([0;-strainrate*u(1)].* DuDx); 
% --------------------------------------------------------------
function u0 = pdex5ic(x)
tw=0;
u0 = [tw; 0]; 
% --------------------------------------------------------------
function [pl,ql,pr,qr] = pdex5bc(xl,ul,xr,ur,t)
pl = [0; ul(2)]; 
ql = [1; 0]; 
pr = [0;ur(2)-1]; 
qr = [1; 0];

Here I am getting some negative values of u1. And the values are also fluctuating. I want to minimize the fluctuation and keep the value of u1 always positive.