Oracle Message ORA-27411 - empty string is not a valid repeat interval.

ERRORempty string is not a valid repeat interval.
CAUSEAn empty string was provided as a repeat interval for a window or a schedule.
ACTIONSpecify a valid repeat interval, e.g., FREQ=DAILY;BYHOUR=8;BYMINUTE=0;BYSECOND=0 for a job that executes daily at 8am.