diff --git a/pgxn/neon/walproposer.c b/pgxn/neon/walproposer.c
index 1f7c473e7d..171af7d2aa 100644
--- a/pgxn/neon/walproposer.c
+++ b/pgxn/neon/walproposer.c
@@ -959,8 +959,8 @@ DetermineEpochStartLsn(WalProposer *wp)
 	}
 
 	/*
-	 * If propEpochStartLsn is 0 everywhere, we are bootstrapping -- nothing
-	 * was committed yet. Start streaming then from the basebackup LSN.
+	 * If propEpochStartLsn is 0, it means flushLsn is 0 everywhere, we are bootstrapping
+	 * and nothing was committed yet. Start streaming then from the basebackup LSN.
 	 */
 	if (wp->propEpochStartLsn == InvalidXLogRecPtr && !wp->config->syncSafekeepers)
 	{
@@ -973,12 +973,13 @@ DetermineEpochStartLsn(WalProposer *wp)
 	}
 
 	/*
-	 * If propEpochStartLsn is not 0, at least one msg with WAL was sent to
-	 * some connected safekeeper; it must have carried truncateLsn pointing to
-	 * the first record.
+	 * Safekeepers are setting truncateLsn after timelineStartLsn is known, so it
+	 * should never be zero at this point, if we know timelineStartLsn.
+	 * 
+	 * timelineStartLsn can be zero only on the first syncSafekeepers run.
 	 */
 	Assert((wp->truncateLsn != InvalidXLogRecPtr) ||
-		   (wp->config->syncSafekeepers && wp->truncateLsn == wp->propEpochStartLsn));
+		   (wp->config->syncSafekeepers && wp->truncateLsn == wp->timelineStartLsn));
 
 	/*
 	 * We will be generating WAL since propEpochStartLsn, so we should set
diff --git a/safekeeper/src/safekeeper.rs b/safekeeper/src/safekeeper.rs
index f63e8576ad..d66db9b652 100644
--- a/safekeeper/src/safekeeper.rs
+++ b/safekeeper/src/safekeeper.rs
@@ -742,6 +742,11 @@ where
                     state.timeline_start_lsn
                 );
             }
+            if state.peer_horizon_lsn == Lsn(0) {
+                // Update peer_horizon_lsn as soon as we know where timeline starts.
+                // It means that peer_horizon_lsn cannot be zero after we know timeline_start_lsn.
+                state.peer_horizon_lsn = msg.timeline_start_lsn;
+            }
             if state.local_start_lsn == Lsn(0) {
                 state.local_start_lsn = msg.start_streaming_at;
                 info!("setting local_start_lsn to {:?}", state.local_start_lsn);