SQL & PL/SQL

I'm trying to check value of variable in case statement in following code:

declare
v_switch varchar2(9) ;
begin
case v_switch
  when v_switch = 'one' then dbms_output.put_line('one');
  when v_switch = 'two' then dbms_output.put_line('two');
  when v_switch = 'three' then dbms_output.put_line('three');
  else dbms_output.put_line('null');
  end case;
end;

but oracle returns:

Error report -

ORA-06550: line 5, column 6:

PLS-00615: type mismatch found at 'V_SWITCH' between CASE operand and WHEN operands

What caused this error?