Quartus Prime Lite: Error in WHEN conditional

This question is very basic, but I just started learning VHDL, and I can't figure out the error.So, I'm using Quartus Prime Lite, and getting error

library ieee;
use ieee.std_logic_1164.all;
package Package_42 is
    subtype StepType is std_logic_vector(3 downto 0);
    function Problem_42(a   : in std_logic;
                        b   :in std_logic;
                        j   : in StepType;
                        k   : in StepType) return StepType;
end;
package body Package_42 is
   function Problem_42(a   : in std_logic;
                       b   : in std_logic;
                       j   : in StepType;
                       k   : in StepType) return StepType is
        variable Step : StepType    := "----";
   begin
       Step :="0100" when a = '1' and b = '0' else -- ERROR is HERE!!!
               j      when a = '1'else
               k      when            b = '1' else
               "----";
        return Step;
   end;
end package body;