How Return String Value From Stored Procedure

This procedure gives a error : " Msg 245, Level 16, State 1, Procedure YAMAN, Line 16
Conversion failed when converting the nvarchar value 'user' to data type int. "
How can i return string value

ALTER procedure [dbo].[YAMAN]
(@username varchar(20),@active varchar(20))
as
begin
if exists (select username from aspnet_Users where username=@username)
begin
if @active=(select active from aspnet_Users where username=@username)
return 'already exist'
else
begin
update aspnet_Users set active=@active where username=@username
return 'update'
end
end
else
return 'user does not exist'
end

Yaman