Unrecognized function or variable 'pendulum_mex_solver'


I installed acados according to the documentation on Windows (for use with Matlab), Automatic build of acados (minGW) by running acados_install_windows().
then i try a matlab example, run the minimal_example_ocp, and it threw this error message:

compiling D:\MatlabR2021b\AddToolBox\acados\examples\acados_matlab_octave....\interfaces\acados_matlab_octave\ocp_get_cost.c
Building with ‘MinGW64 Compiler (C)’.
MEX completed successfully.
compiling D:\MatlabR2021b\AddToolBox\acados\examples\acados_matlab_octave....\interfaces\acados_matlab_octave\ocp_get.c
Building with ‘MinGW64 Compiler (C)’.
MEX completed successfully.
compiling D:\MatlabR2021b\AddToolBox\acados\examples\acados_matlab_octave....\interfaces\acados_matlab_octave\ocp_eval_param_sens.c
Building with ‘MinGW64 Compiler (C)’.
MEX completed successfully.
acados MEX interface compiled successfully
Successfully rendered acados templates!
Unrecognized function or variable ‘pendulum_mex_solver’.

Error in acados_ocp (line 209)
obj.t_ocp = mex_solver();

Error in minimal_example_ocp (line 110)
ocp = acados_ocp(ocp_model, ocp_opts, simulink_opts);

but i found there do have a pendulum_mex_solver.m classdef file in the c_generated_code folder, then why does it throw a error :

Unrecognized function or variable ‘pendulum_mex_solver’

and i tried to add the c_generated_code folder to the path, but it didn’t work.

so what the reason is? thank you for help.