diff --git a/.pylintrc b/.pylintrc
index b2424cdb4..1fd037012 100644
--- a/.pylintrc
+++ b/.pylintrc
@@ -34,6 +34,8 @@ disable =
     no-else-return,
     # Discourages small interfaces
     too-few-public-methods,
+    # Too much old code
+    consider-using-f-string,
 
 [REPORTS]
 msg-template = {path}:{line:3d},{column}: {msg} ({symbol})
diff --git a/src/garage/experiment/experiment.py b/src/garage/experiment/experiment.py
index 101540a0c..7eeb5372e 100644
--- a/src/garage/experiment/experiment.py
+++ b/src/garage/experiment/experiment.py
@@ -457,7 +457,7 @@ def dump_json(filename, data):
 
     """
     pathlib.Path(os.path.dirname(filename)).mkdir(parents=True, exist_ok=True)
-    with open(filename, 'w') as f:
+    with open(filename, 'w', encoding='utf-8') as f:
         # We do our own circular reference handling.
         # Sometimes sort_keys fails because the keys don't get made into
         # strings early enough.
@@ -635,53 +635,60 @@ def _default_inner(self, o):
         # of ValueError on a circular reference.
         try:
             return json.JSONEncoder.default(self, o)
-        except TypeError as err:
-            if isinstance(o, dict):
-                data = {}
-                for (k, v) in o.items():
-                    if isinstance(k, str):
-                        data[k] = self.default(v)
-                    else:
-                        data[repr(k)] = self.default(v)
-                return data
-            elif isinstance(o, weakref.ref):
-                return repr(o)
-            elif type(o).__module__.split('.')[0] in self.BLOCKED_MODULES:
-                return repr(o)
-            elif isinstance(o, type):
-                return {'$typename': o.__module__ + '.' + o.__name__}
-            elif isinstance(o, np.number):
-                # For some reason these aren't natively considered
-                # serializable.
-                # JSON doesn't actually have ints, so always use a float.
-                return float(o)
-            elif isinstance(o, np.bool8):
-                return bool(o)
-            elif isinstance(o, enum.Enum):
-                return {
-                    '$enum':
-                    o.__module__ + '.' + o.__class__.__name__ + '.' + o.name
-                }
-            elif isinstance(o, np.ndarray):
-                return repr(o)
-            elif hasattr(o, '__dict__') or hasattr(o, '__slots__'):
-                obj_dict = getattr(o, '__dict__', None)
-                if obj_dict is not None:
-                    data = {k: self.default(v) for (k, v) in obj_dict.items()}
-                else:
-                    data = {
-                        s: self.default(getattr(o, s))
-                        for s in o.__slots__
+        except TypeError:
+            try:
+                if isinstance(o, dict):
+                    data = {}
+                    for (k, v) in o.items():
+                        if isinstance(k, str):
+                            data[k] = self.default(v)
+                        else:
+                            data[repr(k)] = self.default(v)
+                    return data
+                elif isinstance(o, weakref.ref):
+                    return repr(o)
+                elif type(o).__module__.split('.')[0] in self.BLOCKED_MODULES:
+                    return repr(o)
+                elif isinstance(o, type):
+                    return {'$typename': o.__module__ + '.' + o.__name__}
+                elif isinstance(o, np.number):
+                    # For some reason these aren't natively considered
+                    # serializable.
+                    # JSON doesn't actually have ints, so always use a float.
+                    # Some strange numpy "number" types can actually be None,
+                    # so this case can actually fail as well.
+                    return float(o)
+                elif isinstance(o, np.bool8):
+                    return bool(o)
+                elif isinstance(o, enum.Enum):
+                    return {
+                        '$enum':
+                        o.__module__ + '.' + o.__class__.__name__ + '.' +
+                        o.name
                     }
-                t = type(o)
-                data['$type'] = t.__module__ + '.' + t.__name__
-                return data
-            elif callable(o) and hasattr(o, '__name__'):
-                if getattr(o, '__module__', None) is not None:
-                    return {'$function': o.__module__ + '.' + o.__name__}
-                else:
+                elif isinstance(o, np.ndarray):
                     return repr(o)
-            else:
+                elif hasattr(o, '__dict__') or hasattr(o, '__slots__'):
+                    obj_dict = getattr(o, '__dict__', None)
+                    if obj_dict is not None:
+                        data = {
+                            k: self.default(v)
+                            for (k, v) in obj_dict.items()
+                        }
+                    else:
+                        data = {
+                            s: self.default(getattr(o, s))
+                            for s in o.__slots__
+                        }
+                    t = type(o)
+                    data['$type'] = t.__module__ + '.' + t.__name__
+                    return data
+                elif callable(o) and hasattr(o, '__name__'):
+                    if getattr(o, '__module__', None) is not None:
+                        return {'$function': o.__module__ + '.' + o.__name__}
+                    else:
+                        return repr(o)
+            except TypeError:
                 try:
                     # This case handles many built-in datatypes like deques
                     return [self.default(v) for v in list(o)]
@@ -692,4 +699,4 @@ def _default_inner(self, o):
                     return repr(o)
                 except TypeError:
                     pass
-                raise err
+            return '$invalid'
diff --git a/tests/garage/experiment/test_log_encoder.py b/tests/garage/experiment/test_log_encoder.py
new file mode 100644
index 000000000..90be35061
--- /dev/null
+++ b/tests/garage/experiment/test_log_encoder.py
@@ -0,0 +1,15 @@
+import json
+
+import numpy as np
+
+from garage.experiment.experiment import LogEncoder
+
+
+def test_encode_none_timedelta64():
+    value = {'test': np.timedelta64(None)}
+    encoded = json.dumps(value,
+                         indent=2,
+                         sort_keys=False,
+                         cls=LogEncoder,
+                         check_circular=False)
+    assert 'test' in encoded