diff --git a/stix2/datastore/__init__.py b/stix2/datastore/__init__.py
index 4e53b17..e0de6fe 100644
--- a/stix2/datastore/__init__.py
+++ b/stix2/datastore/__init__.py
@@ -359,7 +359,7 @@ class DataSource(with_metaclass(ABCMeta)):
 
         return results
 
-    def related_to(self, obj, relationship_type=None, source_only=False, target_only=False, filters=[]):
+    def related_to(self, obj, relationship_type=None, source_only=False, target_only=False, filters=None):
         """Retrieve STIX Objects that have a Relationship involving the given
         STIX object.
 
diff --git a/stix2/datastore/filters.py b/stix2/datastore/filters.py
index a116507..10bbeee 100644
--- a/stix2/datastore/filters.py
+++ b/stix2/datastore/filters.py
@@ -44,27 +44,35 @@ def _check_filter_components(prop, op, value):
     return True
 
 
-def _assemble_filters(filter_arg, filters=[]):
+def _assemble_filters(filters1=None, filters2=None):
     """Assemble a list of filters.
 
     This can be used to allow certain functions to work correctly no matter if
     the user provides a single filter or a list of them.
 
     Args:
-        filter_arg (Filter or list): The single Filter or list of Filters to be
-            coerced into a list of Filters.
-        filters (list, optional): A list of Filters to be automatically appended.
+        filters1 (Filter or list, optional): The single Filter or list of Filters to
+            coerce into a list of Filters.
+        filters2 (Filter or list, optional): The single Filter or list of Filters to
+            append to the list of Filters.
 
     Returns:
         List of Filters.
 
     """
-    if isinstance(filter_arg, list):
-        filters.extend(filter_arg)
+    if filters1 is None:
+        filter_list = []
+    elif not isinstance(filters1, list):
+        filter_list = [filters1]
     else:
-        filters.append(filter_arg)
+        filter_list = filters1
 
-    return filters
+    if isinstance(filters2, list):
+        filter_list.extend(filters2)
+    elif filters2 is not None:
+        filter_list.append(filters2)
+
+    return filter_list
 
 
 class Filter(collections.namedtuple("Filter", ['property', 'op', 'value'])):
diff --git a/stix2/test/test_datastore.py b/stix2/test/test_datastore.py
index e80e8d8..8f40401 100644
--- a/stix2/test/test_datastore.py
+++ b/stix2/test/test_datastore.py
@@ -4,7 +4,7 @@ from taxii2client import Collection
 from stix2 import Filter, MemorySink, MemorySource
 from stix2.datastore import (CompositeDataSource, DataSink, DataSource,
                              make_id, taxii)
-from stix2.datastore.filters import apply_common_filters
+from stix2.datastore.filters import _assemble_filters, apply_common_filters
 from stix2.utils import deduplicate
 
 COLLECTION_URL = 'https://example.com/api1/collections/91a7b528-80eb-42ed-a74d-c6fbd5a26116/'
@@ -473,6 +473,15 @@ def test_filters7():
     assert len(resp) == 1
 
 
+def test_assemble_filters():
+    filter1 = Filter("name", "=", "Malicious site hosting downloader")
+    filter2 = Filter("modified", ">", "2017-01-28T13:49:53.935Z")
+    result = _assemble_filters(filter1, filter2)
+    assert len(result) == 2
+    assert result[0].property == 'name'
+    assert result[1].property == 'modified'
+
+
 def test_deduplicate():
     unique = deduplicate(STIX_OBJS1)
 
diff --git a/stix2/workbench.py b/stix2/workbench.py
index 61e99af..9e31b50 100644
--- a/stix2/workbench.py
+++ b/stix2/workbench.py
@@ -148,7 +148,7 @@ _setup_workbench()
 # Functions to get all objects of a specific type
 
 
-def attack_patterns(filters=[]):
+def attack_patterns(filters=None):
     """Retrieve all Attack Pattern objects.
 
     Args:
@@ -160,7 +160,7 @@ def attack_patterns(filters=[]):
     return query(filter_list)
 
 
-def campaigns(filters=[]):
+def campaigns(filters=None):
     """Retrieve all Campaign objects.
 
     Args:
@@ -172,7 +172,7 @@ def campaigns(filters=[]):
     return query(filter_list)
 
 
-def courses_of_action(filters=[]):
+def courses_of_action(filters=None):
     """Retrieve all Course of Action objects.
 
     Args:
@@ -184,7 +184,7 @@ def courses_of_action(filters=[]):
     return query(filter_list)
 
 
-def identities(filters=[]):
+def identities(filters=None):
     """Retrieve all Identity objects.
 
     Args:
@@ -196,7 +196,7 @@ def identities(filters=[]):
     return query(filter_list)
 
 
-def indicators(filters=[]):
+def indicators(filters=None):
     """Retrieve all Indicator objects.
 
     Args:
@@ -208,7 +208,7 @@ def indicators(filters=[]):
     return query(filter_list)
 
 
-def intrusion_sets(filters=[]):
+def intrusion_sets(filters=None):
     """Retrieve all Intrusion Set objects.
 
     Args:
@@ -220,7 +220,7 @@ def intrusion_sets(filters=[]):
     return query(filter_list)
 
 
-def malware(filters=[]):
+def malware(filters=None):
     """Retrieve all Malware objects.
 
     Args:
@@ -232,7 +232,7 @@ def malware(filters=[]):
     return query(filter_list)
 
 
-def observed_data(filters=[]):
+def observed_data(filters=None):
     """Retrieve all Observed Data objects.
 
     Args:
@@ -244,7 +244,7 @@ def observed_data(filters=[]):
     return query(filter_list)
 
 
-def reports(filters=[]):
+def reports(filters=None):
     """Retrieve all Report objects.
 
     Args:
@@ -256,7 +256,7 @@ def reports(filters=[]):
     return query(filter_list)
 
 
-def threat_actors(filters=[]):
+def threat_actors(filters=None):
     """Retrieve all Threat Actor objects.
 
     Args:
@@ -268,7 +268,7 @@ def threat_actors(filters=[]):
     return query(filter_list)
 
 
-def tools(filters=[]):
+def tools(filters=None):
     """Retrieve all Tool objects.
 
     Args:
@@ -280,7 +280,7 @@ def tools(filters=[]):
     return query(filter_list)
 
 
-def vulnerabilities(filters=[]):
+def vulnerabilities(filters=None):
     """Retrieve all Vulnerability objects.
 
     Args: