c# - SQL returning DBnull instead of integer -

i'm trying add record table, , when record exists, want return -3. in c# code, sql server returns dbnull. not sure why, because returns scope_identity no problem. here sql, appreciated.

alter procedure [dbo].[inserttype]      @name varchar(20),     @username varchar(12),     @returnid int output begin     -- set nocount on added prevent result sets     -- interfering select statements.     set nocount on;       if exists (select name tbltypes                 username = @username , name = @name)     begin         return -3 --already exists     end     else     begin         insert tbltypes (name, username)          values (@name, @username)          set @returnid = scope_identity()         return @returnid     end end