diff --git a/library/x509write_crt.c b/library/x509write_crt.c
index 4725c5f..bac849f 100644
--- a/library/x509write_crt.c
+++ b/library/x509write_crt.c
@@ -545,7 +545,7 @@
         if (c - buf < 1) {
             return MBEDTLS_ERR_X509_BUFFER_TOO_SMALL;
         }
-        *(c--) = 0x0;
+        *(--c) = 0x0;
         len++;
         MBEDTLS_ASN1_CHK_ADD(len, mbedtls_asn1_write_len(&c, buf,
                                                          ctx->serial_len + 1));
diff --git a/tests/data_files/Makefile b/tests/data_files/Makefile
index 388b0ce..340d951 100644
--- a/tests/data_files/Makefile
+++ b/tests/data_files/Makefile
@@ -972,6 +972,12 @@
 
 server1.crt: server1.key server1.req.sha256 $(test_ca_crt) $(test_ca_key_file_rsa)
 	$(MBEDTLS_CERT_WRITE) request_file=server1.req.sha256 issuer_crt=$(test_ca_crt) issuer_key=$(test_ca_key_file_rsa) issuer_pwd=$(test_ca_pwd_rsa) version=1 not_before=20190210144406 not_after=20290210144406 md=SHA1 version=3 output_file=$@
+server1.long_serial.crt: server1.long_serial.crt: server1.key server1.req.sha256 $(test_ca_crt) $(test_ca_key_file_rsa)
+	echo "112233445566778899aabbccddeeff0011223344" > test-ca.server1.tmp.serial
+	$(OPENSSL) ca -in server1.req.sha256 -key PolarSSLTest -config test-ca.server1.test_serial.opensslconf -notext -batch -out $@
+server1.80serial.crt: server1.long_serial.crt: server1.key server1.req.sha256 $(test_ca_crt) $(test_ca_key_file_rsa)
+	echo "8011223344" > test-ca.server1.tmp.serial
+	$(OPENSSL) ca -in server1.req.sha256 -key PolarSSLTest -config test-ca.server1.test_serial.opensslconf -notext -batch -out $@
 server1.noauthid.crt: server1.key server1.req.sha256 $(test_ca_crt) $(test_ca_key_file_rsa)
 	$(MBEDTLS_CERT_WRITE) request_file=server1.req.sha256 issuer_crt=$(test_ca_crt) issuer_key=$(test_ca_key_file_rsa) issuer_pwd=$(test_ca_pwd_rsa) not_before=20190210144406 not_after=20290210144406 md=SHA1 authority_identifier=0 version=3 output_file=$@
 server1.crt.der: server1.crt
diff --git a/tests/data_files/server1.80serial.crt b/tests/data_files/server1.80serial.crt
new file mode 100644
index 0000000..3ce8570
--- /dev/null
+++ b/tests/data_files/server1.80serial.crt
@@ -0,0 +1,20 @@
+-----BEGIN CERTIFICATE-----
+MIIDRDCCAiygAwIBAgIGAIARIjNEMA0GCSqGSIb3DQEBBQUAMDsxCzAJBgNVBAYT
+Ak5MMREwDwYDVQQKDAhQb2xhclNTTDEZMBcGA1UEAwwQUG9sYXJTU0wgVGVzdCBD
+QTAeFw0xOTAyMTAxNDQ0MDZaFw0yOTAyMTAxNDQ0MDZaMDwxCzAJBgNVBAYTAk5M
+MREwDwYDVQQKDAhQb2xhclNTTDEaMBgGA1UEAwwRUG9sYXJTU0wgU2VydmVyIDEw
+ggEiMA0GCSqGSIb3DQEBAQUAA4IBDwAwggEKAoIBAQCpAh89QGrVVVOL/TbugmUu
+FWFeib+46EWQ2+6IFlLT8UNQR5YSWWSHa/0r4Eb5c77dz5LhkVvtZqBviSl5RYDQ
+g2rVQUN3Xzl8CQRHgrBXOXDto+wVGR6oMwhHwQVCqf1Mw7Tf3QYfTRBRQGdzEw9A
++G2BJV8KsVPGMH4VOaz5Wu5/kp6mBVvnE5eFtSOS2dQkBtUJJYl1B92mGo8/CRm+
+rWUsZOuVm9z+QV4XptpsW2nMAroULBYknErczdD3Umdz8S2gI/1+9DHKLXDKiQsE
+2y6mT3Buns69WIniU1meblqSZeKIPwyUGaPd5eidlRPtKdurcBLcWsprF6tSglSx
+AgMBAAGjTTBLMAkGA1UdEwQCMAAwHQYDVR0OBBYEFB901j8pwXR0RTsFEiw9qL1D
+WQKmMB8GA1UdIwQYMBaAFLRa5KWz3tJS9rnVppUP6z68x/3/MA0GCSqGSIb3DQEB
+BQUAA4IBAQBJKeTUCctb/wCxBte2AIiaTfATzukTVtGhKkdy3cY6U2DVSXc+s+jr
+Kut8AYnjp1T6bho98RHbbk+hu+0gBWL2ysJd1+slLBUEotUMTkzgA1YdBXy9J/eM
+HJ2a0ydFll/m2rXx7RRJWSbcgPZxQLDfollnNVfhcb75O3GsT3YfEIsjLmon7NHr
+rJmTp773trg0cNJ6j5dKMA/2SQH5PL1cmcFgNfVZ+etNRIhwpIQYySWJ/468Mcg5
+ZKPY6nubIIj+HPB3Mhy8d9U3gAJvc9iEdzbKjrkJdVROONsyMYge4vnbjyKUr7/m
+ZN1O6pZy9Fvgbdhvx4ZHpfgEsa1qfLCH
+-----END CERTIFICATE-----
diff --git a/tests/data_files/server1.long_serial.crt b/tests/data_files/server1.long_serial.crt
new file mode 100644
index 0000000..1bd6955
--- /dev/null
+++ b/tests/data_files/server1.long_serial.crt
@@ -0,0 +1,20 @@
+-----BEGIN CERTIFICATE-----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+-----END CERTIFICATE-----
diff --git a/tests/data_files/test-ca.server1.test_serial.opensslconf b/tests/data_files/test-ca.server1.test_serial.opensslconf
new file mode 100644
index 0000000..43a520e
--- /dev/null
+++ b/tests/data_files/test-ca.server1.test_serial.opensslconf
@@ -0,0 +1,25 @@
+ [ ca ]
+ default_ca             = test-ca
+
+ [ test-ca ]
+ certificate            = test-ca.crt
+ private_key            = test-ca.key
+ serial                 = test-ca.server1.tmp.serial
+ default_md             = sha1
+ default_startdate      = 20190210144406Z
+ default_enddate        = 20290210144406Z
+ x509_extensions        = v3_ca
+ new_certs_dir          = ./
+ database               = ./test-ca.server1.db
+ policy                 = policy_match
+ unique_subject         = no
+
+ [v3_ca]
+ basicConstraints = CA:false
+ subjectKeyIdentifier=hash
+ authorityKeyIdentifier=keyid:always
+
+ [policy_match]
+ countryName            = supplied
+ organizationName       = supplied
+ commonName             = supplied
diff --git a/tests/suites/test_suite_x509write.data b/tests/suites/test_suite_x509write.data
index a986436..b5e07f5 100644
--- a/tests/suites/test_suite_x509write.data
+++ b/tests/suites/test_suite_x509write.data
@@ -142,6 +142,14 @@
 depends_on:MBEDTLS_RSA_C:MBEDTLS_PKCS1_V15:MBEDTLS_DES_C:MBEDTLS_CIPHER_MODE_CBC:MBEDTLS_HAS_ALG_MD5_VIA_MD_OR_PSA_BASED_ON_USE_PSA:MBEDTLS_USE_PSA_CRYPTO
 x509_crt_check:"data_files/server1.key":"":"C=NL,O=PolarSSL,CN=PolarSSL Server 1":"data_files/test-ca.key":"PolarSSLTest":"C=NL,O=PolarSSL,CN=PolarSSL Test CA":"01":"20190210144406":"20290210144406":MBEDTLS_MD_SHA1:0:0:"NULL":0:0:1:-1:"data_files/server1.ca.crt":2:1:"data_files/test-ca.crt"
 
+Certificate write check Server1 SHA1, Full length serial
+depends_on:MBEDTLS_RSA_C:MBEDTLS_PKCS1_V15:MBEDTLS_DES_C:MBEDTLS_CIPHER_MODE_CBC:MBEDTLS_HAS_ALG_MD5_VIA_MD_OR_PSA_BASED_ON_USE_PSA
+x509_crt_check:"data_files/server1.key":"":"C=NL,O=PolarSSL,CN=PolarSSL Server 1":"data_files/test-ca.key":"PolarSSLTest":"C=NL,O=PolarSSL,CN=PolarSSL Test CA":"112233445566778899aabbccddeeff0011223344":"20190210144406":"20290210144406":MBEDTLS_MD_SHA1:0:0:"NULL":0:0:1:-1:"data_files/server1.long_serial.crt":0:0:"data_files/test-ca.crt"
+
+Certificate write check Server1 SHA1, Serial starting with 0x80
+depends_on:MBEDTLS_RSA_C:MBEDTLS_PKCS1_V15:MBEDTLS_DES_C:MBEDTLS_CIPHER_MODE_CBC:MBEDTLS_HAS_ALG_MD5_VIA_MD_OR_PSA_BASED_ON_USE_PSA
+x509_crt_check:"data_files/server1.key":"":"C=NL,O=PolarSSL,CN=PolarSSL Server 1":"data_files/test-ca.key":"PolarSSLTest":"C=NL,O=PolarSSL,CN=PolarSSL Test CA":"8011223344":"20190210144406":"20290210144406":MBEDTLS_MD_SHA1:0:0:"NULL":0:0:1:-1:"data_files/server1.80serial.crt":0:0:"data_files/test-ca.crt"
+
 Certificate write check Server5 ECDSA
 depends_on:MBEDTLS_HAS_ALG_SHA_256_VIA_MD_OR_PSA_BASED_ON_USE_PSA:MBEDTLS_ECDSA_C:MBEDTLS_ECDSA_DETERMINISTIC:MBEDTLS_ECP_DP_SECP384R1_ENABLED:MBEDTLS_ECP_DP_SECP256R1_ENABLED
 x509_crt_check:"data_files/server5.key":"":"C=NL,O=PolarSSL,CN=PolarSSL Server 1":"data_files/test-ca2.key":"PolarSSLTest":"C=NL,O=PolarSSL,CN=Polarssl Test EC CA":"01":"20190210144406":"20290210144406":MBEDTLS_MD_SHA256:0:0:"NULL":0:0:1:-1:"data_files/server5.crt":0:0:"data_files/test-ca2.crt"