diff --git a/requests_ratelimiter/requests_ratelimiter.py b/requests_ratelimiter/requests_ratelimiter.py
index bce8028..fa3f75b 100644
--- a/requests_ratelimiter/requests_ratelimiter.py
+++ b/requests_ratelimiter/requests_ratelimiter.py
@@ -7,6 +7,7 @@
 
 from pyrate_limiter import Duration, InMemoryBucket, Limiter, Rate
 from pyrate_limiter.abstracts import AbstractBucket, RateItem
+from pyrate_limiter.limiter import combined_lock
 from requests import PreparedRequest, Response, Session, exceptions
 from requests.adapters import HTTPAdapter
 
@@ -146,16 +147,17 @@ def _fill_bucket(self, request: PreparedRequest):
             logger.warning('No buckets available to fill')
             return
 
-        now = bucket.now()
-
         # Use smallest rate interval (first after sorting)
         rate = sorted(bucket.rates, key=lambda r: r.interval)[0]
 
         # Add filler items to saturate the smallest rate limit
         # This ensures we delay before the next request
-        for _ in range(rate.limit):
-            filler_item = RateItem(bucket_name, now, weight=1)
-            bucket.put(filler_item)
+        # Each put() resets bucket.failing_rate, which try_acquire() reads under this lock
+        with combined_lock(self.limiter.lock, blocking=True):
+            now = bucket.now()
+            for _ in range(rate.limit):
+                filler_item = RateItem(bucket_name, now, weight=1)
+                bucket.put(filler_item)
 
     def close(self) -> None:
         """Close the session or adapter and release all rate-limiter resources.
diff --git a/test/test_requests_ratelimiter.py b/test/test_requests_ratelimiter.py
index 35fcb3b..a179c74 100644
--- a/test/test_requests_ratelimiter.py
+++ b/test/test_requests_ratelimiter.py
@@ -4,6 +4,7 @@
 """
 
 import pickle
+from threading import Thread
 from time import sleep
 from unittest.mock import MagicMock, patch
 
@@ -354,6 +355,25 @@ def test_fill_bucket_no_bucket_logs_warning(caplog):
     assert 'No buckets available' in caplog.text
 
 
+def test_fill_bucket_waits_for_limiter_lock():
+    """A filler put() resets bucket.failing_rate, so it must not run while another thread holds
+    the limiter lock between its own put() and waiting()
+    """
+    session = get_mock_session(per_second=5)
+    req = PreparedRequest()
+    req.url = MOCKED_URL
+    bucket = session.limiter.bucket_factory[session._bucket_name(req)]
+
+    filler = Thread(target=session._fill_bucket, args=(req,))
+    with session.limiter.lock:
+        filler.start()
+        filler.join(timeout=0.1)
+        assert bucket.count() == 0
+
+    filler.join()
+    assert bucket.count() == 5
+
+
 @patch_sleep
 def test_custom_bucket_class(mock_sleep):
     class MyBucket(InMemoryBucket):