diff --git a/sbol3/toplevel.py b/sbol3/toplevel.py
index 49c5917..4ceeb7d 100644
--- a/sbol3/toplevel.py
+++ b/sbol3/toplevel.py
@@ -6,6 +6,11 @@
 class TopLevel(Identified):
 
     def __init__(self, identity: str, type_uri: str) -> None:
+        # Sanity check identity, which is required for a TopLevel
+        # More checking on identity happens in Identified, but Identified
+        # does not require an identity, only TopLevel does.
+        if not identity or not isinstance(identity, str):
+            raise ValueError('Identity must be a non-empty string')
         super().__init__(identity, type_uri)
         self.attachments = ReferencedObject(self, SBOL_HAS_ATTACHMENT, 0, math.inf)
 
diff --git a/test/test_custom.py b/test/test_custom.py
index cab29e0..57f9311 100644
--- a/test/test_custom.py
+++ b/test/test_custom.py
@@ -79,6 +79,17 @@ def test_round_trip(self):
         # Compare specially
         self.assertCountEqual([True, False], obj2.foo_bool)
 
+    def test_none_identity(self):
+        # Make sure a ValueError is raised if None is passed
+        # as a CustomTopLevel identity. And also if identity
+        # is an empty string or not a string.
+        with self.assertRaises(ValueError):
+            obj = CustomTopClass(None)
+        with self.assertRaises(ValueError):
+            obj = CustomTopClass('')
+        with self.assertRaises(ValueError):
+            obj = CustomTopClass(3)
+
 
 class TestCustomIdentified(unittest.TestCase):