diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index 28daf72a503..0a97b1d37fb 100644
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -9523,13 +9523,13 @@ GetWALAvailability(XLogRecPtr targetLSN)
 	if (XLogRecPtrIsInvalid(targetLSN))
 		return WALAVAIL_INVALID_LSN;
 
-	currpos = GetXLogWriteRecPtr();
-
 	/*
-	 * calculate the oldest segment currently reserved by all slots,
-	 * considering wal_keep_segments and max_slot_wal_keep_size
+	 * Calculate the oldest segment currently reserved by all slots,
+	 * considering wal_keep_segments and max_slot_wal_keep_size.  Initialize
+	 * oldestSlotSeg to the current segment.
 	 */
-	XLByteToSeg(targetLSN, targetSeg, wal_segment_size);
+	currpos = GetXLogWriteRecPtr();
+	XLByteToSeg(currpos, oldestSlotSeg, wal_segment_size);
 	KeepLogSeg(currpos, &oldestSlotSeg);
 
 	/*
@@ -9548,6 +9548,9 @@ GetWALAvailability(XLogRecPtr targetLSN)
 	else
 		oldestSegMaxWalSize = 1;
 
+	/* the segment we care about */
+	XLByteToSeg(targetLSN, targetSeg, wal_segment_size);
+
 	/*
 	 * No point in returning reserved or extended status values if the
 	 * targetSeg is known to be lost.
@@ -9624,7 +9627,7 @@ KeepLogSeg(XLogRecPtr recptr, XLogSegNo *logSegNo)
 	}
 
 	/* don't delete WAL segments newer than the calculated segment */
-	if (XLogRecPtrIsInvalid(*logSegNo) || segno < *logSegNo)
+	if (segno < *logSegNo)
 		*logSegNo = segno;
 }