How can I estimate a parameter of a nonlinear model?

if true
%%The following script I wrote is called simulate_dynamics.m
L = length(FlowReal);
N = round(L/2);
t_est = Ts*[0:N-1]; 
u_est = FlowReal(1:N)'; % Input to system 
y_est = PressureReal(1:N)'; % Real output to system
simin = [t_est' u_est]; % Input to simulink model
k0 = mean(FlowReal(1:10)); % Initial condition for parameter "k"
K_opt   = fminsearch(@(k) est_err,k0); % Find minimum of quadratic error 
%%The function est_err is used to calculate the quadratic error between simulated output and real output, where the simulated output is depended on the parameter"k". This parameter"k" I want to find with fminsearch()
function error = est_err(k)
global k0 y_est t_est y_sim k t_sim
t_est = t_est';
k = k0;
sim('simpressure',t_sim) % The simulation is a nonlinear system. Its the differential equation with the variable "p" and the time derivative of it: d(p)/dt= 1/k*(u_est+1/(2*b)*(a+sqrt(a^2+4*b*p))). a and b are known constants.The input of the model is simin from workspace and the output of the model is called y_sim and is sent to workspace. 
error = sum((y_sim-y_est).^2);
end
%%The problem I have is when I run my script matlab returns an error message, which I don't understand. I don't know what I'm doing wrong.
Error using reshape
To RESHAPE the number of elements must not change.
Error in timeseries/minus>localminus (line 149)
                  [tsout.DataInfo,tsout.Data] = tsdata.datametadata.defaultminus(...
Error in  -  (line 29)
      tsout = localminus(ts1, ts2, false);
Error in est_err (line 12)
error = sum((y_sim-y_est).^2);
Error in @(k)est_err
Error in fminsearch (line 190)
fv(:,1) = funfcn(x,varargin{:});
Error in simulate_dynamics (line 73)
K_opt   = fminsearch(@(k) est_err,k0);
end