diff --git a/stix2/markings/utils.py b/stix2/markings/utils.py
index 538b18b4..c8c78457 100644
--- a/stix2/markings/utils.py
+++ b/stix2/markings/utils.py
@@ -1,6 +1,6 @@
 """Utility functions for STIX2 data markings."""
 
-import collections
+import collections.abc
 
 from stix2 import exceptions, utils
 
@@ -232,7 +232,7 @@ def iterpath(obj, path=None):
         path.append(varname)
         yield (path, varobj)
 
-        if isinstance(varobj, dict):
+        if isinstance(varobj, collections.abc.Mapping):
 
             for item in iterpath(varobj, path):
                 yield item
@@ -245,7 +245,7 @@ def iterpath(obj, path=None):
 
                 yield (path, item)
 
-                if isinstance(item, dict):
+                if isinstance(item, collections.abc.Mapping):
                     for descendant in iterpath(item, path):
                         yield descendant
 
diff --git a/stix2/test/v21/test_granular_markings.py b/stix2/test/v21/test_granular_markings.py
index ff8fe26d..6d405786 100644
--- a/stix2/test/v21/test_granular_markings.py
+++ b/stix2/test/v21/test_granular_markings.py
@@ -192,6 +192,23 @@ def test_add_marking_mark_same_property_same_marking():
         assert m in after["granular_markings"]
 
 
+def test_add_marking_mark_nested_sub_object_property():
+    # A selector reaching into a STIX sub-object such as external_references
+    # is valid per STIX 2.1 section 7.2.3.1, which gives the example
+    # external_references.[0].source_name.
+    kwargs = MALWARE_KWARGS.copy()
+    kwargs["external_references"] = [
+        {"source_name": "capec", "external_id": "CAPEC-163"},
+    ]
+    before = Malware(**kwargs)
+    selector = "external_references.[0].source_name"
+
+    after = markings.add_markings(before, [MARKING_IDS[0]], [selector])
+
+    assert after["granular_markings"][0]["selectors"] == [selector]
+    assert after["granular_markings"][0]["marking_ref"] == MARKING_IDS[0]
+
+
 @pytest.mark.parametrize(
     "data,marking", [
         (