*** a/src/backend/catalog/index.c
--- b/src/backend/catalog/index.c
***************
*** 1316,1321 **** index_drop(Oid indexId, bool concurrent)
--- 1316,1326 ----
  	 * table lock strong enough to prevent all queries on the table from
  	 * proceeding until we commit and send out a shared-cache-inval notice
  	 * that will make them update their index lists.
+ 	 *
+ 	 * All predicate locks on the index are about to be made invalid. Promote
+ 	 * them to relation locks on the heap. For correctness this must be done
+ 	 * after the index was last seen with indisready = true and before it is
+ 	 * seen with indisvalid = false.
  	 */
  	heapId = IndexGetRelation(indexId, false);
  	if (concurrent)
***************
*** 1349,1354 **** index_drop(Oid indexId, bool concurrent)
--- 1354,1361 ----
  		 */
  		indexRelation = heap_open(IndexRelationId, RowExclusiveLock);
  
+ 		TransferPredicateLocksToHeapRelation(userIndexRelation);
+ 
  		tuple = SearchSysCacheCopy1(INDEXRELID,
  									ObjectIdGetDatum(indexId));
  		if (!HeapTupleIsValid(tuple))
***************
*** 1438,1449 **** index_drop(Oid indexId, bool concurrent)
  		userHeapRelation = heap_open(heapId, ShareUpdateExclusiveLock);
  		userIndexRelation = index_open(indexId, AccessExclusiveLock);
  	}
! 
! 	/*
! 	 * All predicate locks on the index are about to be made invalid. Promote
! 	 * them to relation locks on the heap.
! 	 */
! 	TransferPredicateLocksToHeapRelation(userIndexRelation);
  
  	/*
  	 * Schedule physical removal of the files
--- 1445,1452 ----
  		userHeapRelation = heap_open(heapId, ShareUpdateExclusiveLock);
  		userIndexRelation = index_open(indexId, AccessExclusiveLock);
  	}
! 	else
! 		TransferPredicateLocksToHeapRelation(userIndexRelation);
  
  	/*
  	 * Schedule physical removal of the files