diff --git a/personal_data/activity.py b/personal_data/activity.py
index aed2a29..4f9033b 100644
--- a/personal_data/activity.py
+++ b/personal_data/activity.py
@@ -47,7 +47,7 @@ class RealizedActivitySample(ActivitySample):
 
 def heuristically_realize_samples(
     samples: list[ActivitySample],
-    default_estimated_duration: datetime.timedelta = DEFAULT_ESTIMATED_DURATION,
+    default_estimated_duration: datetime.timedelta | None = DEFAULT_ESTIMATED_DURATION,
 ) -> Iterator[RealizedActivitySample]:
     """Secret sauce.
 
@@ -55,6 +55,9 @@ def heuristically_realize_samples(
     * No samples overlap.
     """
 
+    if default_estimated_duration is None:
+        default_estimated_duration = DEFAULT_ESTIMATED_DURATION
+
     samples.sort(key=lambda x: x.end_at)
 
     previous_sample_end = None