diff --git a/src/libopensc/pkcs15.c b/src/libopensc/pkcs15.c
index ba28f030..b9d9f207 100644
--- a/src/libopensc/pkcs15.c
+++ b/src/libopensc/pkcs15.c
@@ -34,7 +34,7 @@ static const struct sc_asn1_entry c_asn1_twlabel[] = {
 
 static const struct sc_asn1_entry c_asn1_toki[] = {
 	{ "version",        SC_ASN1_INTEGER,      SC_ASN1_TAG_INTEGER, 0, NULL, NULL },
-	{ "serialNumber",   SC_ASN1_OCTET_STRING, SC_ASN1_TAG_OCTET_STRING, 0, NULL, NULL },
+	{ "serialNumber",   SC_ASN1_OCTET_STRING, SC_ASN1_TAG_OCTET_STRING, SC_ASN1_OPTIONAL, NULL, NULL },
 	{ "manufacturerID", SC_ASN1_UTF8STRING,   SC_ASN1_TAG_UTF8STRING, SC_ASN1_OPTIONAL, NULL, NULL },
 	{ "label",	    SC_ASN1_UTF8STRING,   SC_ASN1_CTX | 0, SC_ASN1_OPTIONAL, NULL, NULL },
 	/* XXX the Taiwanese ID card erroneously uses explicit tagging */