diff --git a/safekeeper/src/copy_timeline.rs b/safekeeper/src/copy_timeline.rs
index 14bd3c03b8..220988c3ce 100644
--- a/safekeeper/src/copy_timeline.rs
+++ b/safekeeper/src/copy_timeline.rs
@@ -74,10 +74,16 @@ pub async fn handle_request(request: Request) -> Result<()> {
         assert!(flush_lsn >= start_lsn);
 
         if request.until_lsn > flush_lsn {
-            bail!("requested LSN is beyond the end of the timeline");
+            bail!(format!(
+                "requested LSN {} is beyond the end of the timeline {}",
+                request.until_lsn, flush_lsn
+            ));
         }
         if request.until_lsn < start_lsn {
-            bail!("requested LSN is before the start of the timeline");
+            bail!(format!(
+                "requested LSN {} is before the start of the timeline {}",
+                request.until_lsn, start_lsn
+            ));
         }
 
         if request.until_lsn > commit_lsn {
diff --git a/test_runner/regress/test_wal_acceptor.py b/test_runner/regress/test_wal_acceptor.py
index e0ad4fdd5c..2e906e6160 100644
--- a/test_runner/regress/test_wal_acceptor.py
+++ b/test_runner/regress/test_wal_acceptor.py
@@ -2065,6 +2065,11 @@ def test_timeline_copy(neon_env_builder: NeonEnvBuilder, insert_rows: int):
         log.info(f"Original digest: {orig_digest}")
 
         for sk in env.safekeepers:
+            wait(
+                partial(is_flush_lsn_caught_up, sk, tenant_id, timeline_id, lsn),
+                f"sk_id={sk.id} to flush {lsn}",
+            )
+
             sk.http_client().copy_timeline(
                 tenant_id,
                 timeline_id,