diff --git a/prometheus_client/values.py b/prometheus_client/values.py
index bcc66f72..05331f82 100644
--- a/prometheus_client/values.py
+++ b/prometheus_client/values.py
@@ -1,5 +1,5 @@
 import os
-from threading import Lock
+from threading import RLock
 import warnings
 
 from .mmap_dict import mmap_key, MmapedDict