Make a calculator display an error message for non-numerical values.

I am trying to program a simple calculator with 2 "edit text" windows. I want it to display an error dialog for non-numerical values. I used the following function at the "edit text" gui callback:

endb=str2num(get(hObject, 'String'));
if isnan(b)
    errordlg('Input must be a number', 'Error')
end
handles.edit1=b;
guidata(hObject,handles)

but it only displays the error message if I type in 'nan'. How to make it appear for all non-numerical values?

[Jan]

If you set a breakpoint in the 2nd line, you can check, what STR2NUM replies for your non-numerical value. STR2DOUBLE would be safer.