Funny error when using parameters of the drone in the assignement

I am able to stabilize with a drone through a GUI that requires me to create a controller on Matlab but I have some huge difficulties to make it take off because of a funny error:

Subscript indices must either be real positive integers or logicals.

a MatLab error I don't understand... Has des actually to become an integer? I don't think so ;)

I have tried to create the following equation to deal with the test:

I am okay with the fact that I don't get the first member of the equation right now. But I would be glad if you had any idea how to find it.

My main issue is that even without having this first member I have the error I mentioned above, can you help me debug it?

function [ u ] = pd_controller(~, s, s_des, params)
%PD_CONTROLLER  PD controller for the height
%
%   s: 2x1 vector containing the current state [z; v_z]
%   s_des: 2x1 vector containing desired state [z; v_z]
%   params: robot parameters

u = 0;

K = [10 5];

% z(t) = dsolve(diff(z,t) == t*z)

% FILL IN YOUR CODE HERE
u= params.mass(K*s-K*s_des+params.gravity);

%u = params.mass*(params.gravity);

end

The error log is:

>> runsim
Initializing figures...
Simulation Running....
Subscript indices must either be real positive integers or logicals.

Error in controller (line 17)
u= params.mass(K*s-K*s_des+params.gravity);

Error in sys_eom (line 5)
u_des = controlhandle(t, s, s_des, params);

Error in height_control>@(t,s)sys_eom(t,s,controlhandle,trajhandle,params)

Error in odearguments (line 87)
f0 = feval(ode,t0,y0,args{:});   % ODE15I sets args{1} to yp0.

Error in ode45 (line 115)
    odearguments(FcnHandlesUsed, solver_name, ode, tspan, y0, options, varargin);

Error in height_control (line 80)
  [tsave, xsave] = ode45(@(t,s) sys_eom(t, s, controlhandle, trajhandle, params), timeint, x);

Error in runsim (line 17)
[t, z] = height_control(trajhandle, controlhandle);

Answer 1

[Walter Roberson]

 u = params.mass.*(K*s-K*s_des+params.gravity);