EXECUTE query INTO problem

    I've got a problem with a function: It receives two parameters, the 
first, the table name, and the second, a where condition. The function 
looks like this:
   DECLARE      cod bigint;      query TEXT;
   BEGIN      query = 'SELECT codigo FROM ' || $1 || ' WHERE ' || $2;
      EXECUTE query INTO cod;      ·      ·      ·   END;
   I've alwais get the same error, in the EXECUTE sentence: it says: 
Error at or near NULL at character X
   I've also tried declaring cod as row, but the error is the same.
   Anybody can help me?
   Thank you very much