*** a/src/backend/access/heap/heapam.c
--- b/src/backend/access/heap/heapam.c
***************
*** 190,198 **** static const int MultiXactStatusLock[MaxMultiXactStatus + 1] =
  /* Get the LockTupleMode for a given MultiXactStatus */
  #define TUPLOCK_from_mxstatus(status) \
  			(MultiXactStatusLock[(status)])
- /* Get the is_update bit for a given MultiXactStatus */
- #define ISUPDATE_from_mxstatus(status) \
- 			((status) > MultiXactStatusForUpdate)
  
  /* ----------------------------------------------------------------
   *						 heap support routines
--- 190,195 ----
*** a/src/backend/access/transam/multixact.c
--- b/src/backend/access/transam/multixact.c
***************
*** 457,463 **** MultiXactIdExpand(MultiXactId multi, TransactionId xid, MultiXactStatus status)
  	for (i = 0, j = 0; i < nmembers; i++)
  	{
  		if (TransactionIdIsInProgress(members[i].xid) ||
! 			((members[i].status > MultiXactStatusForUpdate) &&
  			 TransactionIdDidCommit(members[i].xid)))
  		{
  			newMembers[j].xid = members[i].xid;
--- 457,463 ----
  	for (i = 0, j = 0; i < nmembers; i++)
  	{
  		if (TransactionIdIsInProgress(members[i].xid) ||
! 			(ISUPDATE_from_mxstatus(members[i].status) &&
  			 TransactionIdDidCommit(members[i].xid)))
  		{
  			newMembers[j].xid = members[i].xid;
***************
*** 713,718 **** MultiXactIdCreateFromMembers(int nmembers, MultiXactMember *members)
--- 713,734 ----
  		return multi;
  	}
  
+ 	/* Verify that there is a single update Xid among the given members. */
+ 	{
+ 		int			i;
+ 		bool		has_update = false;
+ 
+ 		for (i = 0; i < nmembers; i++)
+ 		{
+ 			if (ISUPDATE_from_mxstatus(members[i].status))
+ 			{
+ 				if (has_update)
+ 					elog(ERROR, "new multixact has more than one updating member");
+ 				has_update = true;
+ 			}
+ 		}
+ 	}
+ 
  	/*
  	 * Assign the MXID and offsets range to use, and make sure there is space
  	 * in the OFFSETs and MEMBERs files.  NB: this routine does
*** a/src/include/access/multixact.h
--- b/src/include/access/multixact.h
***************
*** 48,53 **** typedef enum
--- 48,57 ----
  
  #define MaxMultiXactStatus MultiXactStatusUpdate
  
+ /* does a status value correspond to a tuple update? */
+ #define ISUPDATE_from_mxstatus(status) \
+ 			((status) > MultiXactStatusForUpdate)
+ 
  
  typedef struct MultiXactMember
  {