Can't assign my 'Fly By ' back to F3 key

[signalnorths]

Hi , I recently tried to change my fly by key from F3 to a button on my joystick but it wouldnt work and so tried to turn it back to F3. However when I tried to re-assign it to F3 it showed 'Null' as a key I had pressed. How odd! Any idea what I can do to resolve this please. Thank you all in advance. 

[Zooropa_Fly]

Oh no.. not 'NULL'  

[signalnorths]

That was helpful. Cheers