diff --git a/proto/marshal/rules/stringy_numbers.py b/proto/marshal/rules/stringy_numbers.py
index 0d808cc2..dae69e9c 100644
--- a/proto/marshal/rules/stringy_numbers.py
+++ b/proto/marshal/rules/stringy_numbers.py
@@ -31,7 +31,10 @@ def to_python(self, value, *, absent: bool = None):
         return value
 
     def to_proto(self, value):
-        return self._python_type(value)
+        if value is not None:
+            return self._python_type(value)
+
+        return None
 
 
 class Int64Rule(StringyNumberRule):
diff --git a/tests/test_marshal_stringy_numbers.py b/tests/test_marshal_stringy_numbers.py
new file mode 100644
index 00000000..f3f40831
--- /dev/null
+++ b/tests/test_marshal_stringy_numbers.py
@@ -0,0 +1,50 @@
+# Copyright 2021 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import pytest
+
+from proto.marshal.marshal import BaseMarshal
+from proto.primitives import ProtoType
+
+INT_32BIT_PLUS_ONE = 0xFFFFFFFF + 1
+
+
+@pytest.mark.parametrize(
+    "pb_type,value,expected",
+    [
+        (ProtoType.INT64, 0, 0),
+        (ProtoType.INT64, INT_32BIT_PLUS_ONE, INT_32BIT_PLUS_ONE),
+        (ProtoType.SINT64, -INT_32BIT_PLUS_ONE, -INT_32BIT_PLUS_ONE),
+        (ProtoType.INT64, None, None),
+        (ProtoType.UINT64, 0, 0),
+        (ProtoType.UINT64, INT_32BIT_PLUS_ONE, INT_32BIT_PLUS_ONE),
+        (ProtoType.UINT64, None, None),
+        (ProtoType.SINT64, 0, 0),
+        (ProtoType.SINT64, INT_32BIT_PLUS_ONE, INT_32BIT_PLUS_ONE),
+        (ProtoType.SINT64, -INT_32BIT_PLUS_ONE, -INT_32BIT_PLUS_ONE),
+        (ProtoType.SINT64, None, None),
+        (ProtoType.FIXED64, 0, 0),
+        (ProtoType.FIXED64, INT_32BIT_PLUS_ONE, INT_32BIT_PLUS_ONE),
+        (ProtoType.FIXED64, -INT_32BIT_PLUS_ONE, -INT_32BIT_PLUS_ONE),
+        (ProtoType.FIXED64, None, None),
+        (ProtoType.SFIXED64, 0, 0),
+        (ProtoType.SFIXED64, INT_32BIT_PLUS_ONE, INT_32BIT_PLUS_ONE),
+        (ProtoType.SFIXED64, -INT_32BIT_PLUS_ONE, -INT_32BIT_PLUS_ONE),
+        (ProtoType.SFIXED64, None, None),
+    ],
+)
+def test_marshal_to_proto_stringy_numbers(pb_type, value, expected):
+
+    marshal = BaseMarshal()
+    assert marshal.to_proto(pb_type, value) == expected