Unable to resolve a reference to a variable

Question

I am having an error message telling me that it is unable to a resolve a reference to a variable when I try to query a certain data block. I have debugged the form and have made certain that the correct variables are in the form. Why am I getting this error? Here is my code:
Declare     
     n_proj_id t_assignment.proj_id%TYPE := NULL;
     n_con_id t_consultant.con_id%TYPE;
begin
     n_con_id := :t_consultant.con_id;
     SELECT proj_id
     INTO n_proj_id
     FROM t_consultant
     WHERE t_consultant.con_id = n_con_id;
     IF n_proj_id IS NOT NULL THEN
          Set_Block_Property('t_assignment', ONETIME_WHERE, 'proj_id = :n_proj_id');
          go_block('t_assignment');
          execute_query;
     END IF;
end;

Answer 1

it's the n_proj_id variable that can't be reference. Just wanted to clarify.