diff --git a/tests/test_metrics.py b/tests/test_metrics.py
index bbd2d04..15c4388 100644
--- a/tests/test_metrics.py
+++ b/tests/test_metrics.py
@@ -271,6 +271,77 @@ def mock_query(su_info, su_id):
         assert exporter.storage_capacity.labels(storage_unit_id="SU_002")._value.get() == 500
         assert call_count == 2
 
+    def test_storage_metrics_skips_capacity_when_none(self):
+        """When capacity is None (unlimited storage), capacity/utilization
+        gauges must not be populated, but other metrics still are.
+
+        Regression test guarding against re-introducing ``Gauge.set(None)``,
+        which raises ``TypeError`` inside ``float()`` and floods the logs
+        with warnings once per collection interval.
+        """
+        exporter = TQMetricsExporter()
+
+        fake_su_info = MagicMock()
+        fake_su_info.id = "SU_UNLIMITED"
+        exporter._storage_unit_infos = {"SU_UNLIMITED": fake_su_info}
+
+        exporter._query_storage_unit = MagicMock(
+            return_value={
+                "storage_unit_id": "SU_UNLIMITED",
+                "capacity": None,
+                "active_keys": 42,
+                "process_rss_bytes": 128 * 1024 * 1024,
+            }
+        )
+
+        exporter.collect_storage_metrics()
+
+        # capacity / utilization must have no series for this storage unit
+        assert ("SU_UNLIMITED",) not in exporter.storage_capacity._metrics
+        assert ("SU_UNLIMITED",) not in exporter.storage_utilization._metrics
+        # active_keys and memory_rss are still reported
+        assert exporter.storage_active_keys.labels(storage_unit_id="SU_UNLIMITED")._value.get() == 42
+        assert exporter.storage_memory_rss.labels(storage_unit_id="SU_UNLIMITED")._value.get() == 128 * 1024 * 1024
+
+    def test_storage_metrics_prunes_stale_capacity_on_switch_to_unlimited(self):
+        """If a storage unit transitions from a numeric capacity to unlimited,
+        the previously-set ``storage_capacity`` / ``storage_utilization``
+        series must be removed so dashboards do not keep serving stale values.
+        """
+        exporter = TQMetricsExporter()
+
+        fake_su_info = MagicMock()
+        fake_su_info.id = "SU_SWITCH"
+        exporter._storage_unit_infos = {"SU_SWITCH": fake_su_info}
+
+        # First collection: numeric capacity → both gauges get populated.
+        exporter._query_storage_unit = MagicMock(
+            return_value={
+                "storage_unit_id": "SU_SWITCH",
+                "capacity": 1000,
+                "active_keys": 250,
+                "process_rss_bytes": 64 * 1024 * 1024,
+            }
+        )
+        exporter.collect_storage_metrics()
+        assert exporter.storage_capacity.labels(storage_unit_id="SU_SWITCH")._value.get() == 1000
+        assert exporter.storage_utilization.labels(storage_unit_id="SU_SWITCH")._value.get() == 0.25
+
+        # Second collection: unlimited capacity → stale series must be pruned.
+        exporter._query_storage_unit = MagicMock(
+            return_value={
+                "storage_unit_id": "SU_SWITCH",
+                "capacity": None,
+                "active_keys": 300,
+                "process_rss_bytes": 64 * 1024 * 1024,
+            }
+        )
+        exporter.collect_storage_metrics()
+        assert ("SU_SWITCH",) not in exporter.storage_capacity._metrics
+        assert ("SU_SWITCH",) not in exporter.storage_utilization._metrics
+        # active_keys keeps updating with the latest value.
+        assert exporter.storage_active_keys.labels(storage_unit_id="SU_SWITCH")._value.get() == 300
+
 
 # ---------------------------------------------------------------------------
 # Test: ZMQ request type registration
diff --git a/transfer_queue/metrics.py b/transfer_queue/metrics.py
index 97d1018..8d7d7b5 100644
--- a/transfer_queue/metrics.py
+++ b/transfer_queue/metrics.py
@@ -332,9 +332,26 @@ def collect_storage_metrics(self) -> None:
                 label = metrics.get("storage_unit_id", su_id)
                 capacity = metrics.get("capacity", 0)
                 active = metrics.get("active_keys", 0)
-                self.storage_capacity.labels(storage_unit_id=label).set(capacity)
+                # ``capacity`` is ``None`` when the storage unit is configured with
+                # unlimited capacity (``total_storage_size=None``).
+                if capacity is not None:
+                    self.storage_capacity.labels(storage_unit_id=label).set(capacity)
+                    self.storage_utilization.labels(storage_unit_id=label).set(
+                        active / capacity if capacity > 0 else 0.0
+                    )
+                else:
+                    # If the storage unit was previously reporting a numeric
+                    # capacity and later transitions to unlimited (e.g. after
+                    # re-registration), remove the stale series so dashboards
+                    # do not keep serving outdated capacity / utilization
+                    # values. This mirrors the stale-label pruning applied to
+                    # partition-level gauges above.
+                    for gauge in (self.storage_capacity, self.storage_utilization):
+                        try:
+                            gauge.remove(label)
+                        except (KeyError, ValueError):
+                            pass
                 self.storage_active_keys.labels(storage_unit_id=label).set(active)
-                self.storage_utilization.labels(storage_unit_id=label).set(active / capacity if capacity > 0 else 0.0)
                 self.storage_memory_rss.labels(storage_unit_id=label).set(metrics.get("process_rss_bytes", 0))
 
                 # Per-operation request stats