diff --git a/VERSION b/VERSION
index 437459c..73462a5 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-2.5.0
+2.5.1
diff --git a/metrics/legacy_matomo/opensearch_actions.py b/metrics/legacy_matomo/opensearch_actions.py
index 7adaa95..847d321 100644
--- a/metrics/legacy_matomo/opensearch_actions.py
+++ b/metrics/legacy_matomo/opensearch_actions.py
@@ -59,7 +59,10 @@ def increment = params.document.containsKey(field) ? params.document[field] : 0;
   ctx._source.applied_day_masks = params.empty_day_masks;
 }
 for (int index = 0; index < params.day_masks.size(); index++) {
-  if ((ctx._source.applied_day_masks[index] & params.day_masks[index]) != 0) {
+  // JSON zeroes may deserialize as Integer; force long before using high bits.
+  long currentMask = ((Number) ctx._source.applied_day_masks[index]).longValue();
+  long incomingMask = ((Number) params.day_masks[index]).longValue();
+  if ((currentMask & incomingMask) != 0) {
     throw new IllegalStateException('Historical migration overlaps an applied analytics day');
   }
 }
@@ -78,7 +81,9 @@ def increment = params.document.containsKey(field) ? params.document[field] : 0;
   ctx._source[field] = currentValue + increment;
 }
 for (int index = 0; index < params.day_masks.size(); index++) {
-  ctx._source.applied_day_masks[index] |= params.day_masks[index];
+  long currentMask = ((Number) ctx._source.applied_day_masks[index]).longValue();
+  long incomingMask = ((Number) params.day_masks[index]).longValue();
+  ctx._source.applied_day_masks[index] = currentMask | incomingMask;
 }
 ctx._source.applied_migrations.add(params.migration_id);
 """
diff --git a/metrics/tests/services/test_legacy_matomo_importer.py b/metrics/tests/services/test_legacy_matomo_importer.py
index 4ff250c..4c022ef 100644
--- a/metrics/tests/services/test_legacy_matomo_importer.py
+++ b/metrics/tests/services/test_legacy_matomo_importer.py
@@ -50,6 +50,22 @@ def test_analytics_action_builds_all_day_masks(self):
             day_masks[august_last_offset // 63] & (1 << (august_last_offset % 63))
         )
 
+    def test_analytics_action_preserves_masks_larger_than_32_bits(self):
+        action = build_analytics_increment_action(
+            "usage_yearly_analytics_scl_2025",
+            "key",
+            METRICS,
+            "migration-with-hash",
+            ["2025-09-01", "2025-09-30"],
+        )
+
+        day_masks = action["script"]["params"]["day_masks"]
+        script = action["script"]["source"]
+
+        self.assertGreater(max(day_masks), (1 << 31) - 1)
+        self.assertIn("((Number) params.day_masks[index]).longValue()", script)
+        self.assertNotIn("applied_day_masks[index] |=", script)
+
     @override_settings(OPENSEARCH_INDEX_NAME="usage")
     def test_preflight_builds_yearly_targets_without_writes(self):
         collection = SimpleNamespace(