logical decoding - reading a user catalog table

My logical decoding plugin is occasionally getting this  error

"could not resolve cmin/cmax of catalog tuple"

I get this when my output plugin is trying to read one of the user 
defined catalog tables (user_catalog_table=true)


I am not sure if this is a bug in the time-travel support in the logical 
decoding support of if I'm just using it wrong (ie not getting a 
sufficient lock on the relation or something).



This is the interesting part of the stack trace

#4  0x000000000091bbc8 in HeapTupleSatisfiesHistoricMVCC 
(htup=0x7fffcf42a900,    snapshot=0x7f786ffe92d8, buffer=10568) at tqual.c:1631
#5  0x00000000004aedf3 in heapgetpage (scan=0x28d7080, page=0) at 
heapam.c:399
#6  0x00000000004b0182 in heapgettup_pagemode (scan=0x28d7080,    dir=ForwardScanDirection, nkeys=0, key=0x0) at
heapam.c:747
#7  0x00000000004b1ba6 in heap_getnext (scan=0x28d7080,    direction=ForwardScanDirection) at heapam.c:1475
#8  0x00007f787002dbfb in lookupSlonyInfo (tableOid=91754, ctx=0x2826118,    origin_id=0x7fffcf42ab8c,
table_id=0x7fffcf42ab88,
 
set_id=0x7fffcf42ab84)    at slony_logical.c:663
#9  0x00007f787002b7a3 in pg_decode_change (ctx=0x2826118, txn=0x28cbec0,    relation=0x7f787a3446a8,
change=0x7f786ffe3268)at slony_logical.c:237
 
#10 0x00000000007497d4 in change_cb_wrapper (cache=0x28cbda8, 
txn=0x28cbec0,    relation=0x7f787a3446a8, change=0x7f786ffe3268) at logical.c:704



Here is what the code in lookupSlonyInfo is doing
------------------
  sltable_oid = get_relname_relid("sl_table",slony_namespace);
  sltable_rel = relation_open(sltable_oid,AccessShareLock);  tupdesc=RelationGetDescr(sltable_rel);
scandesc=heap_beginscan(sltable_rel,
 
GetCatalogSnapshot(sltable_oid),0,NULL);  reloid_attnum = get_attnum(sltable_oid,"tab_reloid");
  if(reloid_attnum == InvalidAttrNumber)  elog(ERROR,"sl_table does not have a tab_reloid column");  set_attnum =
get_attnum(sltable_oid,"tab_set");
  if(set_attnum == InvalidAttrNumber)  elog(ERROR,"sl_table does not have a tab_set column");  tableid_attnum =
get_attnum(sltable_oid,"tab_id");
 
  if(tableid_attnum == InvalidAttrNumber)  elog(ERROR,"sl_table does not have a tab_id column");
  while( (tuple = heap_getnext(scandesc,ForwardScanDirection) ))