oracle error - ORA-00236: snapshot operation disallowed: mounted control file is a backup

ORA-00236:	snapshot operation disallowed: mounted control file is a backup
Cause:	Attempting to invoke cfileSetSnapshotName, cfileMakeAndUseSnapshot, or cfileUseSnapshot when the currently mounted control file is a backup control file.
Action:	Mount a current control file and retry the operation.