Generating a sine signal with time dependent frequency in Matlab

I want to generate a sine signal y(t) with a time dependent frequency f(t) in Matlab.

I've already tried to realise this using the sine function of Matlab:

h = 0.0001;
npoints = 150/h;
for i = 1:1:npoints
   f(i) = 2 - 0.01*i*h;
   y(i) = 0.5*sin(2*3.1415*f(i)*i*h)+0.5;
end

where the frequency is decreasing with time and h is the time step width.

My problem:

The signal y(t) doesn't look I expected it to look like. There appears a bump in the amplitude at a distinct time (have a look at the plot below).

Does anyone know why this happens and how to generate this sine signal correctly?