diff --git a/init2winit/trainer_lib/base_trainer.py b/init2winit/trainer_lib/base_trainer.py
index 456dfe7d..31b57966 100644
--- a/init2winit/trainer_lib/base_trainer.py
+++ b/init2winit/trainer_lib/base_trainer.py
@@ -31,6 +31,7 @@
 from init2winit.trainer_lib import training_algorithm
 from init2winit.training_metrics_grabber import make_training_metrics
 import jax
+from ml_collections.config_dict import config_dict
 import orbax.checkpoint as ocp
 
 
@@ -192,7 +193,11 @@ def __init__(
     self._xm_work_unit = None
     if callback_configs is None:
       self._callback_configs = []
-    elif isinstance(callback_configs, dict):
+    elif isinstance(callback_configs, (dict, config_dict.ConfigDict)):
+      # Wrap a single callback config dict (or ConfigDict) into a list.
+      # Workload definitions that set callback_configs as a bare dict have it
+      # automatically promoted to a ConfigDict by ml_collections, so we must
+      # handle both types here.
       self._callback_configs = [callback_configs]
     else:
       self._callback_configs = callback_configs