diff --git a/src/backend/access/transam/multixact.c b/src/backend/access/transam/multixact.c
index 5532dc6fc82..424cceb1cf0 100644
--- a/src/backend/access/transam/multixact.c
+++ b/src/backend/access/transam/multixact.c
@@ -2514,7 +2514,8 @@ DetermineSafeOldestOffset(MultiXactId oldestMXact)
 	 */
 	oldestOffset = find_multixact_start(oldestMXact);
 	/* move back to start of the corresponding segment */
-	oldestOffset -= oldestOffset / MULTIXACT_MEMBERS_PER_PAGE * SLRU_PAGES_PER_SEGMENT;
+	oldestOffset -= oldestOffset %
+		(MULTIXACT_MEMBERS_PER_PAGE * SLRU_PAGES_PER_SEGMENT);
 
 	LWLockAcquire(MultiXactGenLock, LW_EXCLUSIVE);
 	/* always leave one segment before the wraparound point */