Log based replication does not work when sourcing standby database

[mpcarter]

Using Oracle 12c Data Guard database as source, got this error when attempting LOG_BASED replication.

tap-oracle | INFO BEGIN
tap-oracle |                          DBMS_LOGMNR.START_LOGMNR(
tap-oracle |                                  startScn => 104799366143,
tap-oracle |                                  endScn => 104799427983,
tap-oracle |                                  OPTIONS => DBMS_LOGMNR.DICT_FROM_ONLINE_CATALOG +
tap-oracle |                                             DBMS_LOGMNR.COMMITTED_DATA_ONLY +
tap-oracle |                                             DBMS_LOGMNR.CONTINUOUS_MINE);
tap-oracle |                          END;
tap-oracle | CRITICAL ORA-01300: writable database required for specified LogMiner options
tap-oracle | ORA-06512: at "SYS.DBMS_LOGMNR", line 58