diff --git a/example/config.py b/example/config.py
index 1eade47..b0eac80 100644
--- a/example/config.py
+++ b/example/config.py
@@ -10,4 +10,6 @@
 _C.TRAIN.HYPERPARAMETER_1 = 0.1
 _C.TRAIN.SCALES = (2, 4, 8, 16)
 
+_C.DATASETS = CN(new_allowed=True)
+
 cfg = _C
diff --git a/example/config.yaml b/example/config.yaml
index d1e9fdd..a352f21 100644
--- a/example/config.yaml
+++ b/example/config.yaml
@@ -2,3 +2,7 @@ SYSTEM:
   NUM_GPUS: 2
 TRAIN:
   SCALES: (1, 2)
+DATASETS:
+  train_2017:
+    17: 1
+    18: 1
diff --git a/yacs/config.py b/yacs/config.py
index e39898d..54bec35 100644
--- a/yacs/config.py
+++ b/yacs/config.py
@@ -129,7 +129,7 @@ def _create_config_tree_from_dict(cls, dic, key_list):
                 _assert_with_logging(
                     _valid_type(v, allow_cfg_node=False),
                     "Key {} with value {} is not a valid type; valid types: {}".format(
-                        ".".join(key_list + [k]), type(v), _VALID_TYPES
+                        ".".join(key_list + [str(k)]), type(v), _VALID_TYPES
                     ),
                 )
         return dic