diff --git a/python/ray/tune/logger.py b/python/ray/tune/logger.py
index 2aa30ceb5..6095cfb4d 100644
--- a/python/ray/tune/logger.py
+++ b/python/ray/tune/logger.py
@@ -5,10 +5,12 @@ from __future__ import print_function
 import csv
 import json
 import logging
-import numpy as np
 import os
 import yaml
 import distutils.version
+import numbers
+
+import numpy as np
 
 import ray.cloudpickle as cloudpickle
 from ray.tune.log_sync import get_syncer
@@ -255,11 +257,19 @@ class _SafeFallbackEncoder(json.JSONEncoder):
     def default(self, value):
         try:
             if np.isnan(value):
-                return None
-            if np.issubdtype(value, float):
-                return float(value)
-            if np.issubdtype(value, int):
+                return self.nan_str
+
+            if (type(value).__module__ == np.__name__
+                    and isinstance(value, np.ndarray)):
+                return value.tolist()
+
+            if issubclass(type(value), numbers.Integral):
                 return int(value)
+            if issubclass(type(value), numbers.Number):
+                return float(value)
+
+            return super(_SafeFallbackEncoder, self).default(value)
+
         except Exception:
             return str(value)  # give up, just stringify it (ok for logs)