diff --git a/py/src/braintrust/prompt_cache/test_disk_cache.py b/py/src/braintrust/prompt_cache/test_disk_cache.py
index 7f49986e..64af065a 100644
--- a/py/src/braintrust/prompt_cache/test_disk_cache.py
+++ b/py/src/braintrust/prompt_cache/test_disk_cache.py
@@ -202,6 +202,36 @@ def test_serializer_handles_complex_objects(self):
 
         self.assertEqual(result.as_dict(), test_prompt.as_dict())
 
+    def test_prompt_with_string_function_call_round_trips(self):
+        cache = disk_cache.DiskCache[prompt.PromptSchema](
+            cache_dir=self.cache_dir,
+            serializer=lambda value: value.as_dict(),
+            deserializer=prompt.PromptSchema.from_dict_deep,
+        )
+        test_prompt = prompt.PromptSchema(
+            id="prompt-id",
+            project_id="project-id",
+            _xact_id="version",
+            name="templated function call",
+            slug="templated-call",
+            description=None,
+            tags=None,
+            prompt_data=prompt.PromptData(
+                prompt=prompt.PromptChatBlock(
+                    messages=[prompt.PromptMessage(role="assistant", content="", function_call="{{function_call}}")]
+                )
+            ),
+        )
+        cache.set("test-key", test_prompt)
+
+        result = cache.get("test-key")
+
+        self.assertEqual(result.as_dict(), test_prompt.as_dict())
+        block = result.prompt_data.prompt
+        assert isinstance(block, prompt.PromptChatBlock)
+        self.assertIsInstance(block.messages[0], prompt.PromptMessage)
+        self.assertEqual(block.messages[0].function_call, "{{function_call}}")
+
     def test_throw_on_deserializer_error(self):
         """Test that deserializer errors are propagated."""
 
diff --git a/py/src/braintrust/serializable_data_class.py b/py/src/braintrust/serializable_data_class.py
index ba32ecca..84af4839 100644
--- a/py/src/braintrust/serializable_data_class.py
+++ b/py/src/braintrust/serializable_data_class.py
@@ -45,7 +45,7 @@ def from_dict_deep(cls, d: dict):
                     if t == type(None) and v is None:
                         filtered[k] = None
                         break
-                    if isinstance(t, type) and issubclass(t, SerializableDataClass) and v is not None:
+                    if isinstance(t, type) and issubclass(t, SerializableDataClass) and isinstance(v, dict):
                         try:
                             filtered[k] = t.from_dict_deep(v)
                             break
diff --git a/py/src/braintrust/test_serializable_data_class.py b/py/src/braintrust/test_serializable_data_class.py
index e31e2078..fc97cc1f 100644
--- a/py/src/braintrust/test_serializable_data_class.py
+++ b/py/src/braintrust/test_serializable_data_class.py
@@ -1,6 +1,7 @@
 import unittest
 from dataclasses import dataclass
 
+from .prompt import FunctionCall, PromptMessage
 from .serializable_data_class import SerializableDataClass
 
 
@@ -23,6 +24,24 @@ class PromptSchema(SerializableDataClass):
 
 
 class TestSerializableDataClass(unittest.TestCase):
+    def test_union_preserves_string_and_none_values(self):
+        for function_call in ("{{function_call}}", None):
+            with self.subTest(function_call=function_call):
+                message = PromptMessage.from_dict_deep(
+                    {"role": "assistant", "content": "", "function_call": function_call}
+                )
+                self.assertEqual(message.function_call, function_call)
+
+    def test_union_deserializes_nested_dataclass(self):
+        message = PromptMessage.from_dict_deep(
+            {
+                "role": "assistant",
+                "content": "",
+                "function_call": {"name": "lookup", "arguments": "{}"},
+            }
+        )
+        self.assertEqual(message.function_call, FunctionCall(name="lookup", arguments="{}"))
+
     def test_from_dict_deep_with_none_values(self):
         """Test that from_dict_deep correctly handles None values in nested objects."""
         test_dict = {