diff --git a/write_buffer/src/core.rs b/write_buffer/src/core.rs
index 2566f2e376..97e22ac8e9 100644
--- a/write_buffer/src/core.rs
+++ b/write_buffer/src/core.rs
@@ -164,6 +164,7 @@ pub mod test_utils {
         test_sequencer_auto_creation(&adapter).await;
         test_sequencer_ids(&adapter).await;
         test_span_context(&adapter).await;
+        test_unknown_sequencer_write(&adapter).await;
     }
 
     /// Test IO with a single writer and single reader stream.
@@ -696,6 +697,26 @@ pub mod test_utils {
         assert_span_context_eq(actual_context_2, &span_context_2);
     }
 
+    /// Test that writing to an unknown sequencer produces an error
+    async fn test_unknown_sequencer_write<T>(adapter: &T)
+    where
+        T: TestAdapter,
+    {
+        let context = adapter.new_context(NonZeroU32::try_from(1).unwrap()).await;
+
+        let entry = lp_to_entry("upc user=1 100");
+
+        let writer = context.writing(true).await.unwrap();
+
+        // flip bits to get an unknown sequencer
+        let sequencer_id = !set_pop_first(&mut writer.sequencer_ids()).unwrap();
+
+        writer
+            .store_entry(&entry, sequencer_id, None)
+            .await
+            .unwrap_err();
+    }
+
     /// Assert that the content of the reader is as expected.
     ///
     /// This will read `expected.len()` from the reader and then ensures that the stream is pending.
diff --git a/write_buffer/src/mock.rs b/write_buffer/src/mock.rs
index ec3d999a0e..ce636904cd 100644
--- a/write_buffer/src/mock.rs
+++ b/write_buffer/src/mock.rs
@@ -241,7 +241,11 @@ impl WriteBufferWriting for MockBufferForWriting {
     ) -> Result<(Sequence, Time), WriteBufferError> {
         let mut guard = self.state.entries.lock();
         let entries = guard.as_mut().unwrap();
-        let sequencer_entries = entries.get_mut(&sequencer_id).unwrap();
+        let sequencer_entries = entries
+            .get_mut(&sequencer_id)
+            .ok_or_else::<WriteBufferError, _>(|| {
+                format!("Unknown sequencer: {}", sequencer_id).into()
+            })?;
 
         let sequence_number = sequencer_entries.max_seqno.map(|n| n + 1).unwrap_or(0);