Error: Invalid Input on my program

[HumbGruland]

Hello, and thanks for answering, I'm new to the forum so please tell me if this is the wrong sub-forum.
I recently got an HP Prime, I can't believe how simple it is programming it, however I encountered a few problems when creating a UI for input.

I made a program based on style 2 of:

http://www.hpmuseum.org/forum/thread-455.html

The code for my program is:

Code:


EXPORT PuntoFijo()
BEGIN
 local n,xact,xnue,fx,err,N,f,f1;
 N:=100; xnue:=1;
 if input(
    {f,f1,err,xact},
    "Metodo del punto fijo",
    {"f(X)=", "f(X)=", "Error=", "x0="},
    {
      "Ingrese la funcion rodeada por comillas simples",
      "Ingrese la funcion depsejada rodeada por comillas simples",
      "Ingrese el error maximo",
      "Ingrese la raiz aproximada"
    },
    {f,f1,err,xact}
  ) then
    F1:=f;
    F2:=f1;
    M0:={}; 
    M0(n,1):=n;
    M0(n,2):=xact;
    M0(n,3):=F2(xact);
    M0(n,4):=abs(F1(xact));
    for n from 2 to N+1 do
      xact:=xnue;
      xnue:=F2(xact);
      fx:=F1(xact);
      M0(n,1):=n;
      M0(n,2):=xact;
      M0(n,3):=xnue;
      M0(n,4):=fx;
      if abs(fx)<err then break; end;
    end;
    editMat(M0);
  end;
END;

Basically the same structure, only modified to implement a different numerical method.
The program passes the check.
The problem arises when the program is running; when I try to input the first algebraic function (between simple quotes), I get the message: "Error: Invalid Input", even if I try to input something as simple as a parabola.
Am I doing something wrong?
Is there something my program is missing?
Thanks for your help.