diff --git a/stix2/base.py b/stix2/base.py
index 09b9c39..1c90dab 100644
--- a/stix2/base.py
+++ b/stix2/base.py
@@ -3,12 +3,13 @@
 import collections
 import copy
 import datetime as dt
+
 import json
 
 
 from .exceptions import ExtraFieldsError, ImmutableError, InvalidValueError, \
                         MissingFieldsError, RevokeError, UnmodifiablePropertyError
-from .utils import format_datetime, get_timestamp, NOW
+from .utils import format_datetime, get_timestamp, NOW, parse_into_datetime
 
 __all__ = ['STIXJSONEncoder', '_STIXBase']
 
@@ -58,16 +59,22 @@ class _STIXBase(collections.Mapping):
         if extra_kwargs:
             raise ExtraFieldsError(cls, extra_kwargs)
 
+        # Remove any keyword arguments whose value is None
+        setting_kwargs = {}
+        for prop_name, prop_value in kwargs.items():
+            if prop_value:
+                setting_kwargs[prop_name] = prop_value
+
         # Detect any missing required fields
         required_fields = get_required_properties(cls._properties)
-        missing_kwargs = set(required_fields) - set(kwargs)
+        missing_kwargs = set(required_fields) - set(setting_kwargs)
         if missing_kwargs:
             raise MissingFieldsError(cls, missing_kwargs)
 
         for prop_name, prop_metadata in cls._properties.items():
-            self._check_property(prop_name, prop_metadata, kwargs)
+            self._check_property(prop_name, prop_metadata, setting_kwargs)
 
-        self._inner = kwargs
+        self._inner = setting_kwargs
 
         if self.granular_markings:
             for m in self.granular_markings:
@@ -121,10 +128,14 @@ class _STIXBase(collections.Mapping):
                 unchangable_properties.append(prop)
         if unchangable_properties:
             raise UnmodifiablePropertyError(unchangable_properties)
+        cls = type(self)
         if 'modified' not in kwargs:
             kwargs['modified'] = get_timestamp()
+        else:
+            new_modified_property = parse_into_datetime(kwargs['modified'])
+            if new_modified_property < self.modified:
+                raise InvalidValueError(cls, 'modified', "The new modified datetime cannot be before the current modified datatime.")
         new_obj_inner.update(kwargs)
-        cls = type(self)
         return cls(**new_obj_inner)
 
     def revoke(self):
diff --git a/stix2/exceptions.py b/stix2/exceptions.py
index c04ada4..20e44a7 100644
--- a/stix2/exceptions.py
+++ b/stix2/exceptions.py
@@ -25,7 +25,7 @@ class MissingFieldsError(STIXError, ValueError):
         self.fields = sorted(list(fields))
 
     def __str__(self):
-        msg = "Missing required field(s) for {0}: ({1})."
+        msg = "No values for required field(s) for {0}: ({1})."
         return msg.format(self.cls.__name__,
                           ", ".join(x for x in self.fields))
 
diff --git a/stix2/test/constants.py b/stix2/test/constants.py
index 1c8ae2b..5ef599e 100644
--- a/stix2/test/constants.py
+++ b/stix2/test/constants.py
@@ -39,6 +39,9 @@ RELATIONSHIP_KWARGS = dict(
     target_ref=MALWARE_ID,
 )
 
+# Minimum required args for a Sighting instance
 SIGHTING_KWARGS = dict(
     sighting_of_ref=INDICATOR_ID,
 )
+
+
diff --git a/stix2/test/test_external_reference.py b/stix2/test/test_external_reference.py
index f8d9b66..1e25fd2 100644
--- a/stix2/test/test_external_reference.py
+++ b/stix2/test/test_external_reference.py
@@ -113,4 +113,3 @@ def test_external_reference_source_required():
 
     assert excinfo.value.cls == stix2.ExternalReference
     assert excinfo.value.fields == ["source_name"]
-    assert str(excinfo.value) == "Missing required field(s) for ExternalReference: (source_name)."
diff --git a/stix2/test/test_indicator.py b/stix2/test/test_indicator.py
index 1c2b610..baf4b38 100644
--- a/stix2/test/test_indicator.py
+++ b/stix2/test/test_indicator.py
@@ -93,7 +93,7 @@ def test_indicator_required_fields():
 
     assert excinfo.value.cls == stix2.Indicator
     assert excinfo.value.fields == ["labels", "pattern"]
-    assert str(excinfo.value) == "Missing required field(s) for Indicator: (labels, pattern)."
+    assert str(excinfo.value) == "No values for required field(s) for Indicator: (labels, pattern)."
 
 
 def test_indicator_required_field_pattern():
@@ -102,7 +102,6 @@ def test_indicator_required_field_pattern():
 
     assert excinfo.value.cls == stix2.Indicator
     assert excinfo.value.fields == ["pattern"]
-    assert str(excinfo.value) == "Missing required field(s) for Indicator: (pattern)."
 
 
 def test_indicator_created_ref_invalid_format():
diff --git a/stix2/test/test_kill_chain_phases.py b/stix2/test/test_kill_chain_phases.py
index d2ecc1f..037e930 100644
--- a/stix2/test/test_kill_chain_phases.py
+++ b/stix2/test/test_kill_chain_phases.py
@@ -41,7 +41,6 @@ def test_kill_chain_required_fields():
 
     assert excinfo.value.cls == stix2.KillChainPhase
     assert excinfo.value.fields == ["kill_chain_name", "phase_name"]
-    assert str(excinfo.value) == "Missing required field(s) for KillChainPhase: (kill_chain_name, phase_name)."
 
 
 def test_kill_chain_required_field_chain_name():
@@ -51,7 +50,6 @@ def test_kill_chain_required_field_chain_name():
 
     assert excinfo.value.cls == stix2.KillChainPhase
     assert excinfo.value.fields == ["kill_chain_name"]
-    assert str(excinfo.value) == "Missing required field(s) for KillChainPhase: (kill_chain_name)."
 
 
 def test_kill_chain_required_field_phase_name():
@@ -61,4 +59,3 @@ def test_kill_chain_required_field_phase_name():
 
     assert excinfo.value.cls == stix2.KillChainPhase
     assert excinfo.value.fields == ["phase_name"]
-    assert str(excinfo.value) == "Missing required field(s) for KillChainPhase: (phase_name)."
diff --git a/stix2/test/test_malware.py b/stix2/test/test_malware.py
index edc35b6..f3bfccd 100644
--- a/stix2/test/test_malware.py
+++ b/stix2/test/test_malware.py
@@ -76,7 +76,6 @@ def test_malware_required_fields():
 
     assert excinfo.value.cls == stix2.Malware
     assert excinfo.value.fields == ["labels", "name"]
-    assert str(excinfo.value) == "Missing required field(s) for Malware: (labels, name)."
 
 
 def test_malware_required_field_name():
@@ -85,7 +84,6 @@ def test_malware_required_field_name():
 
     assert excinfo.value.cls == stix2.Malware
     assert excinfo.value.fields == ["name"]
-    assert str(excinfo.value) == "Missing required field(s) for Malware: (name)."
 
 
 def test_cannot_assign_to_malware_attributes(malware):
diff --git a/stix2/test/test_relationship.py b/stix2/test/test_relationship.py
index 72e659c..869107d 100644
--- a/stix2/test/test_relationship.py
+++ b/stix2/test/test_relationship.py
@@ -76,13 +76,16 @@ def test_relationship_id_must_start_with_relationship():
 def test_relationship_required_field_relationship_type():
     with pytest.raises(stix2.exceptions.MissingFieldsError) as excinfo:
         stix2.Relationship()
-    assert str(excinfo.value) == "Missing required field(s) for Relationship: (relationship_type, source_ref, target_ref)."
+    assert excinfo.value.cls == stix2.Relationship
+    assert excinfo.value.fields == ["relationship_type", "source_ref", "target_ref"]
 
 
 def test_relationship_missing_some_required_fields():
     with pytest.raises(stix2.exceptions.MissingFieldsError) as excinfo:
         stix2.Relationship(relationship_type='indicates')
-    assert str(excinfo.value) == "Missing required field(s) for Relationship: (source_ref, target_ref)."
+
+    assert excinfo.value.cls == stix2.Relationship
+    assert excinfo.value.fields == ["source_ref", "target_ref"]
 
 
 def test_relationship_required_field_target_ref():
@@ -94,7 +97,6 @@ def test_relationship_required_field_target_ref():
 
     assert excinfo.value.cls == stix2.Relationship
     assert excinfo.value.fields == ["target_ref"]
-    assert str(excinfo.value) == "Missing required field(s) for Relationship: (target_ref)."
 
 
 def test_cannot_assign_to_relationship_attributes(relationship):
diff --git a/stix2/test/test_versioning.py b/stix2/test/test_versioning.py
index cecaa89..0e51abc 100644
--- a/stix2/test/test_versioning.py
+++ b/stix2/test/test_versioning.py
@@ -23,6 +23,26 @@ def test_making_new_version():
     assert campaign_v1.modified < campaign_v2.modified
 
 
+def test_making_new_version_with_unset():
+    campaign_v1 = stix2.Campaign(
+        id="campaign--8e2e2d2b-17d4-4cbf-938f-98ee46b3cd3f",
+        created_by_ref="identity--f431f809-377b-45e0-aa1c-6a4751cae5ff",
+        created="2016-04-06T20:03:00.000Z",
+        modified="2016-04-06T20:03:00.000Z",
+        name="Green Group Attacks Against Finance",
+        description="Campaign by Green Group against a series of targets in the financial services sector."
+    )
+
+    campaign_v2 = campaign_v1.new_version(description=None)
+
+    assert campaign_v1.id == campaign_v2.id
+    assert campaign_v1.created_by_ref == campaign_v2.created_by_ref
+    assert campaign_v1.created == campaign_v2.created
+    assert campaign_v1.name == campaign_v2.name
+    assert campaign_v2.description == None
+    assert campaign_v1.modified < campaign_v2.modified
+
+
 def test_making_new_version_with_embedded_object():
     campaign_v1 = stix2.Campaign(
         id="campaign--8e2e2d2b-17d4-4cbf-938f-98ee46b3cd3f",
@@ -86,7 +106,42 @@ def test_versioning_error_invalid_property():
     with pytest.raises(stix2.exceptions.UnmodifiablePropertyError) as excinfo:
         campaign_v1.new_version(type="threat-actor")
 
-    str(excinfo.value) == "These properties cannot be changed when making a new version: type"
+    assert str(excinfo.value) == "These properties cannot be changed when making a new version: type."
+
+
+def test_versioning_error_bad_modified_value():
+    campaign_v1 = stix2.Campaign(
+        id="campaign--8e2e2d2b-17d4-4cbf-938f-98ee46b3cd3f",
+        created_by_ref="identity--f431f809-377b-45e0-aa1c-6a4751cae5ff",
+        created="2016-04-06T20:03:00.000Z",
+        modified="2016-04-06T20:03:00.000Z",
+        name="Green Group Attacks Against Finance",
+        description="Campaign by Green Group against a series of targets in the financial services sector."
+    )
+
+    with pytest.raises(stix2.exceptions.InvalidValueError) as excinfo:
+        campaign_v1.new_version(modified="2015-04-06T20:03:00.000Z")
+
+    assert excinfo.value.cls == stix2.Campaign
+    assert excinfo.value.prop_name == "modified"
+    assert excinfo.value.reason == "The new modified datetime cannot be before the current modified datatime."
+
+
+def test_versioning_error_usetting_required_property():
+    campaign_v1 = stix2.Campaign(
+        id="campaign--8e2e2d2b-17d4-4cbf-938f-98ee46b3cd3f",
+        created_by_ref="identity--f431f809-377b-45e0-aa1c-6a4751cae5ff",
+        created="2016-04-06T20:03:00.000Z",
+        modified="2016-04-06T20:03:00.000Z",
+        name="Green Group Attacks Against Finance",
+        description="Campaign by Green Group against a series of targets in the financial services sector."
+    )
+
+    with pytest.raises(stix2.exceptions.MissingFieldsError) as excinfo:
+        campaign_v1.new_version(name=None)
+
+    assert excinfo.value.cls == stix2.Campaign
+    assert excinfo.value.fields == [ "name" ]
 
 
 def test_versioning_error_new_version_of_revoked():
@@ -104,7 +159,7 @@ def test_versioning_error_new_version_of_revoked():
     with pytest.raises(stix2.exceptions.RevokeError) as excinfo:
         campaign_v2.new_version(name="barney")
 
-    str(excinfo.value) == "Cannot create a new version of a revoked object"
+    assert excinfo.value.called_by == "new_version"
 
 
 def test_versioning_error_revoke_of_revoked():
@@ -122,4 +177,4 @@ def test_versioning_error_revoke_of_revoked():
     with pytest.raises(stix2.exceptions.RevokeError) as excinfo:
         campaign_v2.revoke()
 
-    str(excinfo.value) == "Cannot revoke an already revoked object"
+    assert excinfo.value.called_by == "revoke"
diff --git a/stix2/utils.py b/stix2/utils.py
index cdc7f54..5feffae 100644
--- a/stix2/utils.py
+++ b/stix2/utils.py
@@ -1,6 +1,7 @@
 """Utility functions and classes for the stix2 library."""
 
 import datetime as dt
+from dateutil import parser
 import json
 
 import pytz
@@ -34,6 +35,28 @@ def format_datetime(dttm):
     return ts + "Z"
 
 
+def parse_into_datetime(value):
+    if isinstance(value, dt.date):
+        if hasattr(value, 'hour'):
+            return value
+        else:
+            # Add a time component
+            return dt.datetime.combine(value, dt.time(), tzinfo=pytz.utc)
+
+    # value isn't a date or datetime object so assume it's a string
+    try:
+        parsed = parser.parse(value)
+    except TypeError:
+        # Unknown format
+        raise ValueError("must be a datetime object, date object, or "
+                         "timestamp string in a recognizable format.")
+    if parsed.tzinfo:
+        return parsed.astimezone(pytz.utc)
+    else:
+        # Doesn't have timezone info in the string; assume UTC
+        return pytz.utc.localize(parsed)
+
+
 def get_dict(data):
     """Return data as a dictionary.
     Input can be a dictionary, string, or file-like object.