SQL & PL/SQL

I got a table that holds pl/sql code snippets to do validations on a set of data. what the code basically does is receiving a ID and returning a number of errors found.
To execute the code I use dynamic sql with two bind variables.

When the codes consists of a simpel query, it works like a charm, for example with this code:

BEGIN
   SELECT COUNT (1)
   INTO :1
   FROM articles atl
   WHERE ATL.CSE_ID = :2 AND cgp_id IS NULL;
END;

however when I get to some more complex validations that need to do calculations or execute multiple queries, I'm running into trouble.
I've boiled the problem down into this:

DECLARE
   counter   NUMBER;
   my_id     NUMBER := 61;
BEGIN
   EXECUTE IMMEDIATE ('
      declare 
         some_var number;
      begin
      
      select 1 into some_var from dual
      where :2 = 61; 
      
      :1 := :2;
      end;
')
      USING OUT counter, IN my_id;

   DBMS_OUTPUT.put_line (counter || '-' || my_id);
END;

this code doesn't really make any sense, but it's just to show you what the problem is. When I execute this code, I get the error
ORA-6537 OUT bind variable bound to an IN position

The error doesn't seem to make sense, :2 is the only IN bind variable, and it's only used in a where clause.
As soon as I remove that where clause , the code will work again (giving me 61-61, in case you liked to know).

Any idea whats going wrong? Am I just using the bind variables in a way you're not supposed to use them?

I'm using Oracle Database 11g Enterprise Edition Release 11.2.0.3.0 - 64bit