diff --git a/modules/openapi-generator/src/main/resources/python/api_client.mustache b/modules/openapi-generator/src/main/resources/python/api_client.mustache
index 0fe89af0b1c8..fb96a9f03e36 100644
--- a/modules/openapi-generator/src/main/resources/python/api_client.mustache
+++ b/modules/openapi-generator/src/main/resources/python/api_client.mustache
@@ -390,6 +390,10 @@ class ApiClient:
             else:
                 obj_dict = obj.__dict__
 
+        if isinstance(obj_dict, list):
+            # here we handle instances that can either be a list or something else, and only became a real list by calling to_dict()
+            return self.sanitize_for_serialization(obj_dict)
+
         return {
             key: self.sanitize_for_serialization(val)
             for key, val in obj_dict.items()
diff --git a/samples/client/echo_api/python-disallowAdditionalPropertiesIfNotPresent/openapi_client/api_client.py b/samples/client/echo_api/python-disallowAdditionalPropertiesIfNotPresent/openapi_client/api_client.py
index ed46d8c31a22..368214952778 100644
--- a/samples/client/echo_api/python-disallowAdditionalPropertiesIfNotPresent/openapi_client/api_client.py
+++ b/samples/client/echo_api/python-disallowAdditionalPropertiesIfNotPresent/openapi_client/api_client.py
@@ -383,6 +383,10 @@ def sanitize_for_serialization(self, obj):
             else:
                 obj_dict = obj.__dict__
 
+        if isinstance(obj_dict, list):
+            # here we handle instances that can either be a list or something else, and only became a real list by calling to_dict()
+            return self.sanitize_for_serialization(obj_dict)
+
         return {
             key: self.sanitize_for_serialization(val)
             for key, val in obj_dict.items()
diff --git a/samples/client/echo_api/python/openapi_client/api_client.py b/samples/client/echo_api/python/openapi_client/api_client.py
index ed46d8c31a22..368214952778 100644
--- a/samples/client/echo_api/python/openapi_client/api_client.py
+++ b/samples/client/echo_api/python/openapi_client/api_client.py
@@ -383,6 +383,10 @@ def sanitize_for_serialization(self, obj):
             else:
                 obj_dict = obj.__dict__
 
+        if isinstance(obj_dict, list):
+            # here we handle instances that can either be a list or something else, and only became a real list by calling to_dict()
+            return self.sanitize_for_serialization(obj_dict)
+
         return {
             key: self.sanitize_for_serialization(val)
             for key, val in obj_dict.items()
diff --git a/samples/openapi3/client/petstore/python-aiohttp/petstore_api/api_client.py b/samples/openapi3/client/petstore/python-aiohttp/petstore_api/api_client.py
index ab412d976569..077953e623bc 100644
--- a/samples/openapi3/client/petstore/python-aiohttp/petstore_api/api_client.py
+++ b/samples/openapi3/client/petstore/python-aiohttp/petstore_api/api_client.py
@@ -385,6 +385,10 @@ def sanitize_for_serialization(self, obj):
             else:
                 obj_dict = obj.__dict__
 
+        if isinstance(obj_dict, list):
+            # here we handle instances that can either be a list or something else, and only became a real list by calling to_dict()
+            return self.sanitize_for_serialization(obj_dict)
+
         return {
             key: self.sanitize_for_serialization(val)
             for key, val in obj_dict.items()
diff --git a/samples/openapi3/client/petstore/python/petstore_api/api_client.py b/samples/openapi3/client/petstore/python/petstore_api/api_client.py
index 0aad03162b9a..d75ceb2b9a60 100755
--- a/samples/openapi3/client/petstore/python/petstore_api/api_client.py
+++ b/samples/openapi3/client/petstore/python/petstore_api/api_client.py
@@ -382,6 +382,10 @@ def sanitize_for_serialization(self, obj):
             else:
                 obj_dict = obj.__dict__
 
+        if isinstance(obj_dict, list):
+            # here we handle instances that can either be a list or something else, and only became a real list by calling to_dict()
+            return self.sanitize_for_serialization(obj_dict)
+
         return {
             key: self.sanitize_for_serialization(val)
             for key, val in obj_dict.items()