diff --git a/lib/opencdk/write-packet.c b/lib/opencdk/write-packet.c
index 9fb9de9..d698af4 100644
--- a/lib/opencdk/write-packet.c
+++ b/lib/opencdk/write-packet.c
@@ -487,6 +487,9 @@ calc_s2ksize (cdk_pkt_seckey_t sk)
     case CDK_S2K_ITERSALTED:
       nbytes = 11;
       break;
+	case CDK_S2K_GNU_EXT:
+	  nbytes = 2;
+	  break;
     }
   nbytes += sk->protect.ivlen;
   nbytes++;                     /* single cipher byte */