diff --git a/__tests__/shotRepository.test.ts b/__tests__/shotRepository.test.ts
index ba2c501..47b1c5a 100644
--- a/__tests__/shotRepository.test.ts
+++ b/__tests__/shotRepository.test.ts
@@ -183,6 +183,45 @@ describe('shotRepository', () => {
     expect(sessions[0].lastShotAt).toBe('2026-09-14T09:30:00Z');
   });
 
+  // Swing-speed mode reuses the `shot` event but serializes a different key
+  // set: swing_speed_to_shot_dict() omits shot_number entirely rather than
+  // sending it as null (server.py:4322-4374). An absent key deserializes to
+  // undefined, which is not === null, so the identity guard below takes the
+  // wrong branch and undefined reaches the bound SQL parameter. The factory
+  // above always supplies shot_number, so no existing test reaches this path.
+  describe('a shot the server could not number', () => {
+    function makeUnnumberedShot(overrides: Partial<Shot> = {}): Shot {
+      const shot = makeShot(overrides);
+      delete (shot as Partial<Shot>).shot_number;
+      return shot;
+    }
+
+    it('records every swing of a swing-speed session', async () => {
+      // The whole session was being lost, not one shot: undefined reached a
+      // bound parameter, threw, and saveShot's catch swallowed it. Nothing
+      // surfaced, because the live view never reads back from storage.
+      const repo = createShotRepository(openTestDatabase());
+      await repo.init();
+
+      await repo.saveShot('session-1', makeUnnumberedShot({ timestamp: '2026-09-14T10:00:00Z' }));
+      await repo.saveShot('session-1', makeUnnumberedShot({ timestamp: '2026-09-14T10:01:00Z' }));
+
+      expect(await repo.loadShots('session-1')).toHaveLength(2);
+    });
+
+    it('stores an absent shot number as null, the same as an explicit one', async () => {
+      // Normalising on the way in keeps one representation in the database, so
+      // a reloaded shot is indistinguishable from one the server sent as null.
+      const repo = createShotRepository(openTestDatabase());
+      await repo.init();
+
+      await repo.saveShot('session-1', makeUnnumberedShot({ timestamp: '2026-09-14T10:00:00Z' }));
+
+      const [stored] = await repo.loadShots('session-1');
+      expect(stored.shot_number).toBeNull();
+    });
+  });
+
   it('can be initialised twice without losing what is already stored', async () => {
     // init() runs on every launch; a migration that re-ran destructively would
     // wipe the player's history.
diff --git a/__tests__/useSessionStore.test.ts b/__tests__/useSessionStore.test.ts
index 7a74ec2..2215e1f 100644
--- a/__tests__/useSessionStore.test.ts
+++ b/__tests__/useSessionStore.test.ts
@@ -157,3 +157,43 @@ describe('an enriched shot replacing its provisional version', () => {
     expect(useSessionStore.getState().shots).toHaveLength(2);
   });
 });
+
+// Swing-speed mode reuses the `shot` event but serializes a different key set:
+// swing_speed_to_shot_dict() omits shot_number entirely rather than sending it
+// as null (server.py:4322-4374). The null guards below are correct; what was
+// missing is normalising an absent key, which arrives as undefined and is not
+// === null, so it slipped past them. The factories above always supply
+// shot_number, so no existing test reaches this path — these build the payload
+// the way the server actually sends it.
+describe('a shot the server could not number', () => {
+  // Strip the key rather than setting it undefined, so the object matches a
+  // JSON.parse of the real payload.
+  function makeUnnumberedShot(overrides: Partial<Shot> = {}): Shot {
+    const shot = makeShot(overrides);
+    delete (shot as Partial<Shot>).shot_number;
+    return shot;
+  }
+
+  it('follows the same append-never-merge rule as an explicit null', () => {
+    // An absent key and an explicit null both mean "the server could not
+    // number this shot", so both must append. Before this was normalised,
+    // undefined slipped past the null guard and matched the first unnumbered
+    // shot on the list — overwriting a different swing.
+    useSessionStore.getState().addShot(makeUnnumberedShot({ timestamp: 't1', club: 'driver' }));
+
+    useSessionStore.getState().replaceShot(makeUnnumberedShot({ timestamp: 't2', club: '7 iron' }));
+
+    expect(useSessionStore.getState().shots).toHaveLength(2);
+  });
+
+  it('leaves a numbered shot alone when an unnumbered one arrives', () => {
+    // A swing-speed rep must never land on a launch-monitor shot.
+    useSessionStore.getState().addShot(makeShot({ shot_number: 7, club: 'driver' }));
+
+    useSessionStore.getState().replaceShot(makeUnnumberedShot({ timestamp: 't9', club: '7 iron' }));
+
+    const shots = useSessionStore.getState().shots;
+    expect(shots).toHaveLength(2);
+    expect(shots.find((shot) => shot.shot_number === 7)?.club).toBe('driver');
+  });
+});
diff --git a/package.json b/package.json
index 01fde81..8f9472d 100644
--- a/package.json
+++ b/package.json
@@ -60,9 +60,11 @@
     "collectCoverageFrom": [
       "app/**/*.{ts,tsx}",
       "components/**/*.{ts,tsx}",
+      "data/**/*.{ts,tsx}",
       "services/**/*.{ts,tsx}",
       "storage/**/*.{ts,tsx}",
       "stores/**/*.{ts,tsx}",
+      "utils/**/*.{ts,tsx}",
       "types.ts"
     ],
     "coverageReporters": [
diff --git a/storage/shotRepository.ts b/storage/shotRepository.ts
index c0b0f3e..2623afe 100644
--- a/storage/shotRepository.ts
+++ b/storage/shotRepository.ts
@@ -205,17 +205,24 @@ export function createShotRepository(db: ShotDatabase): ShotRepository {
 
     // The single write path for both `shot` and `shot_update`. The server
     // re-emits an enriched shot under the same shot_number, so a shot already
-    // filed is updated in place. A shot the server could not number — nullable
-    // on the wire, though a supported server always sets it — is appended,
-    // which is the old behaviour and cannot collide with anything.
+    // filed is updated in place. A shot the server could not number is
+    // appended, which cannot collide with anything.
+    //
+    // shot_number is normalised before use: swing-speed mode omits the key
+    // from its payload entirely rather than sending null
+    // (swing_speed_to_shot_dict, server.py:4322-4374). The absent key arrives
+    // as undefined, which is not === null, so it slipped past the guard below
+    // and reached a bound SQL parameter — throwing, and being swallowed by the
+    // catch, which discarded every shot of the session.
     async saveShot(sessionId, shot) {
       try {
+        const shotNumber = shot.shot_number ?? null;
         const existing =
-          shot.shot_number === null
+          shotNumber === null
             ? null
             : await db.getFirstAsync<{ id: number }>(
                 'SELECT id FROM shots WHERE session_id = ? AND shot_number = ?',
-                [sessionId, shot.shot_number],
+                [sessionId, shotNumber],
               );
 
         if (existing) {
@@ -231,7 +238,7 @@ export function createShotRepository(db: ShotDatabase): ShotRepository {
         await db.runAsync(
           `INSERT INTO shots (${columns.join(', ')})
            VALUES (${columns.map(() => '?').join(', ')})`,
-          [sessionId, shot.shot_number, ...measurementValues(shot)],
+          [sessionId, shotNumber, ...measurementValues(shot)],
         );
       } catch {
         // A dropped write loses one shot from history; the live view is unaffected.
diff --git a/stores/useSessionStore.ts b/stores/useSessionStore.ts
index b8812a5..d59e54c 100644
--- a/stores/useSessionStore.ts
+++ b/stores/useSessionStore.ts
@@ -52,10 +52,14 @@ export const useSessionStore = create<SessionState>((set) => ({
   addShot: (shot) => set((prev) => ({ shots: [shot, ...prev.shots] })),
   replaceShot: (shot) =>
     set((prev) => {
+      // Normalised because swing-speed payloads omit shot_number rather than
+      // sending null; undefined is not === null and slipped past this guard,
+      // matching every other unnumbered shot on the list.
+      const shotNumber = shot.shot_number ?? null;
       const index =
-        shot.shot_number === null
+        shotNumber === null
           ? -1
-          : prev.shots.findIndex((existing) => existing.shot_number === shot.shot_number);
+          : prev.shots.findIndex((existing) => existing.shot_number === shotNumber);
       if (index === -1) return { shots: [shot, ...prev.shots] };
       const shots = [...prev.shots];
       shots[index] = shot;