both statetements error with ....expected var!(v) to expand to an existing variable or be a part of a match

gistfile1.ex

% from my unit tests

foo = 10

assert 10 == my_macro(:foo)
assert 10 == my_macro("foo")

% macro

defmacro my_macro(some_var) do
  quote do
    v = unquote(some_var)
    if is_atom(v),      do: var!(v)
    if is_binary(v),    do: var!(binary_to_atom(v))        
  end
end