Call function written in the same script using F9

Hello,

let assume this simple code with a custom routine at the end of the code:

a = 0:0.01:100;
b = sin(0:0.01:100);
RMSE_results = Routine_RMSE(a, b);

function [RMSE]=Routine_RMSE(y, yhat)
    RMSE = sqrt(mean((y - yhat).^2));                                      % Root Mean Squared Error
end

If I run it using the RUN button, it works properly.

If I try to run it line by line executing it using F9 the function into the code appears as "undefined", however if I define the routine into an external file, then it works. The function is working also if I execute the entire "section", but is not recognized using F9.

How can call the function written in the same code using F9 ?