mmon session blocking itself?

Oracle 12.1.0.2 SE-2 on Oracle Linux 6, with stand-alone ASM (not RAC, no clustering of any kind)

I have a job that runs every 5 minutes to check for blocking sessions, and send me an email if one is detected.  The core of that code is this query:

for c1 in  (select NVL(s1.username,'null') username1,                s1.machine machine1,                s1.sid sid1,                s1.serial# serial1,                s1.osuser osuser1,                s1.program program1,                lo.object_id locked_object,                NVL(s2.username,'null') username2,                s2.machine machine2,                s2.sid sid2,                s2.serial# serial2,                s2.osuser osuser2,                s2.program program2,                sqlt.sql_text sql_text2         from   v$lock l1,