SQL & PL/SQL

I tried to execute the following function at command line.
CREATE OR REPLACE FUNCTION FN_IS_SYSDATE_TICKING
RETURN BOOLEAN
IS
ls_value VARCHAR2(100) DEFAULT 'NONE';
BEGIN
SELECT value
INTO ls_value
FROM V$PARAMETER
WHERE name = 'fixed_date';

IF ls_value = 'NONE' THEN
RETURN TRUE;
ELSE
RETURN FALSE;
END IF;
EXCEPTION
WHEN OTHERS THEN
RETURN TRUE;
END FN_IS_SYSDATE_TICKING();

At Command Line, I entered

var v1 BOOLEAN;
exec :v1 := RSSC.FN_IS_SYSDATE_TICKING();
print v1;

It is giving boolean is not declared.