diff --git a/backend/app/schemas/job.py b/backend/app/schemas/job.py
index 5934ee9..8be9776 100644
--- a/backend/app/schemas/job.py
+++ b/backend/app/schemas/job.py
@@ -1,6 +1,6 @@
 from typing import Any
 
-from pydantic import BaseModel
+from pydantic import BaseModel, field_validator
 
 from ..models.job import (
     AccessibleVideoProgressItem,
@@ -78,6 +78,11 @@ class VttUpdateRequest(BaseModel):
     if_match: str | None = None  # Optimistic locking — SHA1 of expected current content
     retranslate_languages: bool = False  # Re-translate all target languages from updated source VTT
 
+    @field_validator('captions_vtt', 'audio_description_vtt', mode='before')
+    @classmethod
+    def empty_str_to_none(cls, v: Any) -> str | None:
+        return None if v == '' else v
+
 
 class VttTimingAdjustRequest(BaseModel):
     offset_seconds: float