diff --git a/src/backend/access/transam/varsup.c b/src/backend/access/transam/varsup.c
index 8c3d84fbf23..788b961ef0e 100644
--- a/src/backend/access/transam/varsup.c
+++ b/src/backend/access/transam/varsup.c
@@ -159,7 +159,8 @@ GetNewTransactionId(bool isSubXact)
 
 		/* Re-acquire lock and start over */
 		LWLockAcquire(XidGenLock, LW_EXCLUSIVE);
-		xid = XidFromFullTransactionId(ShmemVariableCache->nextFullXid);
+		full_xid = ShmemVariableCache->nextFullXid;
+		xid = XidFromFullTransactionId(full_xid);
 	}
 
 	/*
diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c
index 816fd626ece..8522a2f51c5 100644
--- a/src/backend/access/transam/xact.c
+++ b/src/backend/access/transam/xact.c
@@ -5151,7 +5151,7 @@ EstimateTransactionStateSpace(void)
 		nxids = add_size(nxids, s->nChildXids);
 	}
 
-	return add_size(size, sizeof(SerializedTransactionState) * nxids);
+	return add_size(size, mul_size(sizeof(TransactionId), nxids));
 }
 
 /*