diff --git a/prometheus_client/metrics.py b/prometheus_client/metrics.py
index 12f793c2..7e06d992 100644
--- a/prometheus_client/metrics.py
+++ b/prometheus_client/metrics.py
@@ -107,7 +107,7 @@ def __init__(self: T,
                  namespace: str = '',
                  subsystem: str = '',
                  unit: str = '',
-                 registry: CollectorRegistry = REGISTRY,
+                 registry: Optional[CollectorRegistry] = REGISTRY,
                  _labelvalues: Optional[Sequence[str]] = None,
                  ) -> None:
         self._name = _build_full_name(self._type, name, namespace, subsystem, unit)
@@ -677,7 +677,7 @@ def __init__(self,
                  namespace: str = '',
                  subsystem: str = '',
                  unit: str = '',
-                 registry: CollectorRegistry = REGISTRY,
+                 registry: Optional[CollectorRegistry] = REGISTRY,
                  _labelvalues: Optional[Sequence[str]] = None,
                  states: Optional[Sequence[str]] = None,
                  ):