Oracle : Ошибка ORA-31670

"Username argument must be specified and non-null."
*Cause: Called change user with a NULL username or a null string.
*Action: Specify a valid username argument. If this error occurs from a
Data Pump client (e.g. expdp or impdp), try the operation again.
If the error occurs again, contact Oracle Customer Support and
report the error.

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