Creating a Certificate Signing Request failed - ssl

Question

I am trying to create a Certificate Signing Request in Rust programmatically with the binding library https://docs.rs/openssl/latest/openssl/index.html.
The following statement creates a CSR with openssl library:
openssl ecparam -name prime256v1 \
-genkey \
-noout \
-out server.key.pem
status=$?
if [ $status -eq 1 ]; then
echo "Creating private key failed"
exit 1
fi
openssl req \
-key server.key.pem \
-subj "/CN=server.acme.io/C=CH/L=Zurich/ST=ZH/O=acme/OU=acme" \
-new -sha256 \
-out server.csr
status=$?
if [ $status -eq 1 ]; then
echo "CSR creation failed"
exit 1
fi
Unfortunately, the following code snippet using the Rust binding library does not work:
use openssl::nid::Nid;
use openssl::ec::{EcGroup, EcKey};
use openssl::error::ErrorStack;
use openssl::x509::{X509NameBuilder, X509ReqBuilder};
use openssl::hash::MessageDigest;
use openssl::pkey::{PKey};
fn main() -> Result<(), ErrorStack> {
let group = EcGroup::from_curve_name(Nid::X9_62_PRIME256V1)?;
let ec = EcKey::generate(&group)?;
let private_key = PKey::from_ec_key(ec)?;
let mut name = X509NameBuilder::new()?;
name.append_entry_by_nid(Nid::COUNTRYNAME, "CH")?;
name.append_entry_by_nid(Nid::STATEORPROVINCENAME, "ZH")?;
name.append_entry_by_nid(Nid::LOCALITYNAME, "Zurich")?;
name.append_entry_by_nid(Nid::ORGANIZATIONALUNITNAME, "ACME")?;
name.append_entry_by_nid(Nid::ORGANIZATIONNAME, "ACME")?;
name.append_entry_by_nid(Nid::COMMONNAME, "acme.io")?;
let mut req = X509ReqBuilder::new()?;
req.set_subject_name(name.build().as_ref())?;
req.sign(private_key.as_ref(), MessageDigest::sha256())?;
Ok(())
}
It shows the error message:
Error: ErrorStack([Error { code: 109052126, library: "asn1 encoding routines", function: "asn1_template_ex_i2d", reason: "illegal zero content", file: "crypto/asn1/tasn_enc.c", line: 374 }, Error { code: 109838595, library: "asn1 encoding routines", function: "ASN1_item_sign_ctx", reason: "internal error", file: "crypto/asn1/a_sign.c", line: 265 }])
What am I doing wrong? All the required fields are filled with values.