Error when setting text column to null value in GridDB (FDW)

Question

The server is terminated connection because of setting null value to text column.

-- join with nullable side with some columns with null values
UPDATE ft5 SET c3 = null where c1 % 9 = 0;
server closed the connection unexpectedly
    This probably means the server terminated abnormally
    before or while processing the request.
connection to server was lost

The schema of table ft5 is below:

CREATE FOREIGN TABLE ft5 (
    c1 int OPTIONS (rowkey 'true'),
    c2 int NOT NULL,
    c3 text
) SERVER griddb_svr OPTIONS (table_name 'T4');

Please help me to fix this issue.