(null) in signed zone file

Using lates source code of ldns and OpenDNSSEC.

I am signing a large zone file (50000 RR generated using the perl script in SVN). First everything went well. But after a couple of hours I got a signed zone with extra rows containing just "(null)". Not all delegations had extra rows.

Example:

label11113.largetld. 3600 IN NS ns1.label11113.largetld.
label11113.largetld. 3600 IN NS ns2.label11113.largetld.
ns1.label11114.largetld. 3600 IN A 192.0.2.1
ns2.label11114.largetld. 3600 IN A 192.0.2.1
label11113.largetld. 60 IN NSEC label11114.largetld. NS RRSIG NSEC
(null)
label11114.largetld. 3600 IN NS ns1.label11114.largetld.
label11114.largetld. 3600 IN NS ns2.label11114.largetld.
ns1.label11115.largetld. 3600 IN A 192.0.2.1
ns2.label11115.largetld. 3600 IN A 192.0.2.1
label11114.largetld. 60 IN NSEC label11115.largetld. NS RRSIG NSEC
(null)
label11115.largetld. 3600 IN NS ns1.label11115.largetld.
label11115.largetld. 3600 IN NS ns2.label11115.largetld.
label11115.largetld. 3600 IN DS 22922 7 1 f62411de95a5b7bcabe976c0e65034a35a9fa937 ; xutid-gygat-vihop-hutur-sypiv-natos-binah-bytyp-fekyn-z$
(null)
ns1.label11116.largetld. 3600 IN A 192.0.2.1
ns2.label11116.largetld. 3600 IN A 192.0.2.1
label11115.largetld. 60 IN NSEC label11116.largetld. NS DS RRSIG NSEC
(null)

But everything was restored when the Signer Engine signed the zone the next time. This bug is random.