diff --git a/pageserver/src/tenant/timeline.rs b/pageserver/src/tenant/timeline.rs
index be72e15c19..8c80a54bdd 100644
--- a/pageserver/src/tenant/timeline.rs
+++ b/pageserver/src/tenant/timeline.rs
@@ -4727,6 +4727,12 @@ impl Timeline {
             return;
         }
 
+        if self.current_logical_size.current_size().is_exact() {
+            // root timelines are initialized with exact count, but never start the background
+            // calculation
+            return;
+        }
+
         if let Some(await_bg_cancel) = self
             .current_logical_size
             .cancel_wait_for_background_loop_concurrency_limit_semaphore
diff --git a/pageserver/src/tenant/timeline/logical_size.rs b/pageserver/src/tenant/timeline/logical_size.rs
index b0d6c4a27a..f4a4eea54a 100644
--- a/pageserver/src/tenant/timeline/logical_size.rs
+++ b/pageserver/src/tenant/timeline/logical_size.rs
@@ -122,6 +122,10 @@ impl CurrentLogicalSize {
             Self::Exact(_) => Accuracy::Exact,
         }
     }
+
+    pub(crate) fn is_exact(&self) -> bool {
+        matches!(self, Self::Exact(_))
+    }
 }
 
 impl LogicalSize {
diff --git a/test_runner/regress/test_timeline_size.py b/test_runner/regress/test_timeline_size.py
index 5e9a42f6b4..1f220eec9e 100644
--- a/test_runner/regress/test_timeline_size.py
+++ b/test_runner/regress/test_timeline_size.py
@@ -936,6 +936,9 @@ def test_timeline_logical_size_task_priority(neon_env_builder: NeonEnvBuilder):
     tenant_id = env.initial_tenant
     timeline_id = env.initial_timeline
 
+    # just make sure this doesn't hit an assertion
+    client.timeline_detail(tenant_id, timeline_id, force_await_initial_logical_size=True)
+
     # load in some data
     endpoint = env.endpoints.create_start("main", tenant_id=tenant_id)
     endpoint.safe_psql_many(