diff --git a/vapi_python/vapi_python.py b/vapi_python/vapi_python.py
index 69e9797..f06c85f 100644
--- a/vapi_python/vapi_python.py
+++ b/vapi_python/vapi_python.py
@@ -38,9 +38,13 @@ def start(
     ):
         # Start a new call
         if assistant_id:
-            payload = {'assistantId': assistant_id, 'assistantOverrides': assistant_overrides}
+            payload = {'assistantId': assistant_id}
+            if assistant_overrides:
+                payload['assistantOverrides'] = assistant_overrides
         elif assistant:
-            payload = {'assistant': assistant, 'assistantOverrides': assistant_overrides}
+            payload = {'assistant': assistant}
+            if assistant_overrides:
+                payload['assistantOverrides'] = assistant_overrides
         elif squad_id:
             payload = {'squadId': squad_id}
         elif squad: