Help with error code for my GUI

I'm currently creating a GUI that is able to graph a user defined function. You are also able to find the zero value at a point by clicking a button that that then activates ginput. The code for this is below.

if true
  % code
[x,y]=ginput(1);
fct=@(x) get(handles.funct,'String');
zero=num2str(fzero(fct,x));
set(handles.edit8,'String',zero);
guidata(hObject,handles)
end

In this code handles.funct is the user defined function, and handles.edit8 is a edit box where the answer will be output. Now when i run this program for a function such as 'cos(x)' i get the following error code as well as the edit box displaying NaN

if true
  % code
Operands to the || and && operators must be
convertible to logical scalar values.
Error in fzero (line 308)
  elseif ~isfinite(fx) || ~isreal(fx)
Error in Matlabfinalproject>findzero_Callback (line
186)
zero=num2str(fzero(fct,x));
Error in gui_mainfcn (line 96)
      feval(varargin{:});
Error in Matlabfinalproject (line 44)
  gui_mainfcn(gui_State, varargin{:});
Error in
@(hObject,eventdata)Matlabfinalproject('findzero_Callback',hObject,eventdata,guidata(hObject))
Error while evaluating uicontrol Callback
end

Not having enough knowledge of how to debug this was wondering if anyone else could help me explain to me why i'm getting this code

[Niklas Nylén]

What does the function fct look like? What is the value of x from ginput?

[Stephen Hawkins]

the user defines fct as x.^3-tan(x)+20. and i'm trying to find a zero at 1