openssl csr 报错maxsize=2

openssl req -new -key private_key.pem -out certificate_request.csr -subj "/C=China/ST=beijing/L=haidian/O=iwall/CN=10.0.20.2" problems making Certificate Request 140557413541528:error:0D07A097:asn1 encoding routines:ASN1_mbstring_ncopy: string too long:a_mbstr.c:158:maxsize=2

C=CN, China 太长了。