ORA-01427: single-row subquery returns more than one row

how to resolve this error?

UPDATE test_instance a
SET TEST_FLOW_SEQUENCE_VERSION =
  (SELECT version
  FROM testflow_report_versions tfrv
  WHERE a.TEST_FLOW_REPORT_ID = tfrv.report_id 
  )
WHERE a.TEST_FLOW_REPORT_ID IN
  (SELECT TEST_FLOW_REPORT_ID
  FROM test_instance
  WHERE TEST_FLOW_REPORT_ID IS NOT NULL
  );

Giving error:
Error report -
SQL Error: ORA-01427: single-row subquery returns more than one row
01427. 00000 -  "single-row subquery returns more than one row"
*Cause:   
*Action: