diff --git a/CHANGELOG.rst b/CHANGELOG.rst
index 47bea3b..c5e7167 100644
--- a/CHANGELOG.rst
+++ b/CHANGELOG.rst
@@ -2,6 +2,11 @@
 CHANGELOG
 =========
 
+0.18.0
+===============
+
+* SumType variants are attribute-strict #48
+
 0.16.0 - 0.17.0
 ===============
 
diff --git a/docs/conf.py b/docs/conf.py
index 06c051b..7214dab 100644
--- a/docs/conf.py
+++ b/docs/conf.py
@@ -51,9 +51,9 @@
 # built documents.
 #
 # The short X.Y version.
-version = '0.17'
+version = '0.18'
 # The full version, including alpha/beta/rc tags.
-release = '0.17.0'
+release = '0.18.0'
 
 # The language for content autogenerated by Sphinx. Refer to documentation
 # for a list of supported languages.
diff --git a/setup.py b/setup.py
index e9a1483..bbd6826 100644
--- a/setup.py
+++ b/setup.py
@@ -38,7 +38,7 @@ def requirements(at_path: Path):
 # ----------------------------
 
 setup(name='typeit',
-      version='0.17.0',
+      version='0.18.0',
       description='typeit brings typed data into your project',
       long_description=README,
       classifiers=[
diff --git a/tests/test_sums.py b/tests/test_sums.py
index 8983f90..34bbfa3 100644
--- a/tests/test_sums.py
+++ b/tests/test_sums.py
@@ -1,4 +1,4 @@
-from typing import Dict
+from typing import Dict, NamedTuple
 
 import pytest
 import pickle
@@ -148,4 +148,26 @@ class VariantB:
         'b': True,
     }
     x = mk_x(data)
-    assert serialize_x(x) == data
\ No newline at end of file
+    assert serialize_x(x) == data
+
+
+def test_sumtype_attr_strictness():
+    class Versioning(SumType):
+        class V1:
+            a: int
+
+        class V2:
+            a: int
+            b: int
+
+    class X(NamedTuple):
+        payload: Versioning
+
+    mk_x, serialize_x = (
+            typeit.type_constructor & typeit.flags.SUM_TYPE_DICT('_version_') ^ X
+    )
+
+    with pytest.raises(typeit.Error):
+        x = mk_x({'payload': {'_version_': 'v1', 'a': 1, 'b': 1}})
+
+    x = mk_x({'payload': {'_version_': 'v2', 'a': 1, 'b': 1}})
diff --git a/typeit/parser.py b/typeit/parser.py
index 8b87437..58768b4 100644
--- a/typeit/parser.py
+++ b/typeit/parser.py
@@ -175,6 +175,7 @@ def _maybe_node_for_sum_type(
                 overrides,
                 memo
             )
+            node.typ.unknown = 'raise'
             variant_nodes.append((variant, node))
         sum_node = schema.nodes.SchemaNode(
             schema.types.Sum(
diff --git a/typeit/utils.py b/typeit/utils.py
index 78f4fd6..5b5758c 100644
--- a/typeit/utils.py
+++ b/typeit/utils.py
@@ -23,9 +23,12 @@ def is_named_tuple(typ: Type[Any]) -> bool:
 def clone_schema_node(node):
     """ Clonning the node and reassigning the same children,
     because clonning is recursive, but we are only interested
-    in new version of the outermost schema node, the children nodes
+    in a new version of the outermost schema node, the children nodes
     should be shared to avoid unnecessary duplicates.
     """
     new_node = node.clone()
-    new_node.children = node.children
-    return new_node
\ No newline at end of file
+    # a list comprehension to place the same nodes into
+    # a new wrapping list object, so that extending
+    # the cloned node with new children doesn't affect the original node
+    new_node.children = [x for x in node.children]
+    return new_node