--- proftpd-dfsg.orig/contrib/mod_tls.c
+++ proftpd-dfsg/contrib/mod_tls.c
@@ -5862,6 +5862,9 @@
       ASN1_INTEGER *sn;
 
       revoked = sk_X509_REVOKED_value(X509_CRL_get_REVOKED(crl), i);
+      if (revoked == NULL) {
+          continue;
+      }
       sn = revoked->serialNumber;
 
       if (ASN1_INTEGER_cmp(sn, X509_get_serialNumber(xs)) == 0) {
