can not create table

[1170300619]

I use this command to prepare data in PostgreSQL database
sysbench ./share/sysbench/tests/include/oltp_legacy/oltp.lua --db-driver=pgsql --pgsql-db=sysbench --pgsql-user=xxx --pgsql-password=xxx --pgsql-port=PORT --pgsql-host=IP --oltp-tables-count=20 --oltp-table-size=2000000 --threads=20 prepare
BUT it doesn't work, and output error message
Creating table 'sbtest1'...

FATAL: PQexec() failed: 7 Constraint conflict for column "c" of table "sbtest1", default value is null, but column is not null
FATAL: failed query was: CREATE TABLE sbtest1 (
id SERIAL NOT NULL,
k INTEGER DEFAULT '0' NOT NULL,
c CHAR(120) DEFAULT '' NOT NULL,
pad CHAR(60) DEFAULT '' NOT NULL,
PRIMARY KEY (id)
) 
FATAL: `prepare' function failed: ./share/sysbench/tests/include/oltp_legacy/common.lua:66: db_query() failed