Debugging with Boolean values:Wrong number or types of arguments in call to '||'

When I use a boolean value with DBMS_OUTPUT.PUT_LINE is get an error.

Error is "Wrong number of types of arguments in call to '||'

This is because DBMS_OUTPUT package is not overloaded for boolean values.

How can I debug boolean values in Pl/Sql then?

***********code***********
Procedure process (p_trade_no IN number,
                   p_ellig_in IN BOOLEAN) AS

BEGIN
      DBMS_OUTPUT.PUT_LINE('value of p_ellig_in is '||p_ellig_in );
      IF p_ellig_in THEN
            null;
      END IF;

END;

Error: Wrong number or types of arguments in call to '||'