diff --git a/src/bentoml/_internal/io_descriptors/pandas.py b/src/bentoml/_internal/io_descriptors/pandas.py
index 84676b501d9..90e44ec69a9 100644
--- a/src/bentoml/_internal/io_descriptors/pandas.py
+++ b/src/bentoml/_internal/io_descriptors/pandas.py
@@ -356,6 +356,8 @@ def _convert_dtype(
             return str(value)
         elif isinstance(value, dict):
             return {str(k): self._convert_dtype(v) for k, v in value.items()}
+        elif value is None:
+            return "null"
         else:
             logger.warning(f"{type(value)} is not yet supported.")
             return None
@@ -822,6 +824,8 @@ def _convert_dtype(
             return str(value)
         elif isinstance(value, dict):
             return {str(k): self._convert_dtype(v) for k, v in value.items()}
+        elif value is None:
+            return "null"
         else:
             logger.warning(f"{type(value)} is not yet supported.")
             return None