diff --git a/frigate/record/export.py b/frigate/record/export.py
index 32df8be75..cf1506c37 100644
--- a/frigate/record/export.py
+++ b/frigate/record/export.py
@@ -420,6 +420,7 @@ class RecordingExporter(threading.Thread):
             return None
 
         total_output = windows[-1][2] + (windows[-1][1] - windows[-1][0])
+        last_recorded_end = windows[-1][1]
 
         def wall_to_output(t: float) -> float:
             t = max(float(self.start_time), min(float(self.end_time), t))
@@ -432,8 +433,18 @@ class RecordingExporter(threading.Thread):
 
         chapter_blocks: list[str] = []
         for review in review_rows:
+            if review.start_time is None:
+                continue
+            # In-progress segments have a NULL end_time until the activity
+            # closes; clamp to the last recorded second so the chapter never
+            # extends past the actual video.
+            review_end = (
+                float(review.end_time)
+                if review.end_time is not None
+                else last_recorded_end
+            )
             start_out = wall_to_output(float(review.start_time))
-            end_out = wall_to_output(float(review.end_time))
+            end_out = wall_to_output(review_end)
 
             # Drop chapters that fall entirely in a recording gap, or are
             # too short to be navigable in a player.
@@ -516,16 +527,14 @@ class RecordingExporter(threading.Thread):
             except DoesNotExist:
                 return ""
 
-            diff = self.start_time - preview.start_time
-            minutes = int(diff / 60)
-            seconds = int(diff % 60)
+            diff = max(0.0, float(self.start_time) - float(preview.start_time))
             ffmpeg_cmd = [
                 "/usr/lib/ffmpeg/7.0/bin/ffmpeg",  # hardcode path for exports thumbnail due to missing libwebp support
                 "-hide_banner",
                 "-loglevel",
                 "warning",
                 "-ss",
-                f"00:{minutes}:{seconds}",
+                f"{diff:.3f}",
                 "-i",
                 preview.path,
                 "-frames",
diff --git a/frigate/test/test_export_progress.py b/frigate/test/test_export_progress.py
index 903914815..62883c13a 100644
--- a/frigate/test/test_export_progress.py
+++ b/frigate/test/test_export_progress.py
@@ -499,5 +499,56 @@ class TestSchedulesCleanup(unittest.TestCase):
             assert job.id not in manager.jobs
 
 
+class TestChapterMetadataInProgressReview(unittest.TestCase):
+    """Regression: in-progress review segments have end_time=NULL until the
+    activity closes. The chapter builder must clamp the chapter end to the
+    last recorded second instead of crashing on float(None)."""
+
+    def _fake_select_returning(self, rows: list) -> MagicMock:
+        mock_query = MagicMock()
+        mock_query.where.return_value = mock_query
+        mock_query.order_by.return_value = mock_query
+        mock_query.iterator.return_value = iter(rows)
+        return mock_query
+
+    def test_in_progress_review_does_not_crash_and_clamps_to_last_recording(
+        self,
+    ) -> None:
+        exporter = _make_exporter(end_minus_start=200)
+        # Recordings cover [1000, 1150]; export window is [1000, 1200] so
+        # the last recorded second is 1150 (a 50s gap at the tail).
+        recordings = [
+            MagicMock(start_time=1000.0, end_time=1150.0),
+        ]
+        in_progress = MagicMock(
+            start_time=1100.0,
+            end_time=None,
+            severity="alert",
+            data={"objects": ["person"]},
+        )
+
+        with tempfile.TemporaryDirectory() as tmpdir:
+            chapter_path = os.path.join(tmpdir, "chapters.txt")
+            exporter._chapter_metadata_path = lambda: chapter_path  # type: ignore[method-assign]
+
+            with patch(
+                "frigate.record.export.ReviewSegment.select",
+                return_value=self._fake_select_returning([in_progress]),
+            ):
+                result = exporter._build_chapter_metadata_file(recordings)
+
+            assert result == chapter_path
+            with open(chapter_path) as f:
+                content = f.read()
+
+        # Output time is windows[-1][1] - windows[-1][0] = 150s.
+        # Review starts at wall=1100, output offset = 100s -> 100000ms.
+        # Clamped end = last_recorded_end (1150) -> output offset = 150s -> 150000ms.
+        assert "[CHAPTER]" in content
+        assert "START=100000" in content
+        assert "END=150000" in content
+        assert "title=Alert: person" in content
+
+
 if __name__ == "__main__":
     unittest.main()