diff --git a/rllib/utils/debug.py b/rllib/utils/debug.py
index 3e44d67fc..24a09302d 100644
--- a/rllib/utils/debug.py
+++ b/rllib/utils/debug.py
@@ -1,5 +1,6 @@
 import numpy as np
 import pprint
+from typing import Mapping
 
 from ray.rllib.policy.sample_batch import SampleBatch, MultiAgentBatch
 
@@ -17,7 +18,7 @@ def summarize(obj):
 
 
 def _summarize(obj):
-    if isinstance(obj, dict):
+    if isinstance(obj, Mapping):
         return {k: _summarize(v) for k, v in obj.items()}
     elif hasattr(obj, "_asdict"):
         return {