arg() crashes the emulator

[Angus]

fun(a):=a^2
arg(fun)

-> crashes the emulator

fun:=a^2
arg(fun)

-> no crash but trapped in a looping error message. Invalid series expansion.

I would expect an error message like argument is nonsense or something instead of a crash or infinit loop.

To avoid an other thread. Given fun(a):=a^2. I have questions.
1) Is it possibly to get the function's argument as I (accidently) tried above: GetArgs(function)
2) defining fun:=(a)->a^2 using the arrow directly gives an error window. Using minus and greater works, but converts both chars to the built in arrow.... strange