diff --git a/sqlmodel/main.py b/sqlmodel/main.py
index 64bad15..de09796 100644
--- a/sqlmodel/main.py
+++ b/sqlmodel/main.py
@@ -148,7 +148,7 @@ def Field(
     unique: bool = False,
     nullable: Union[bool, UndefinedType] = Undefined,
     index: Union[bool, UndefinedType] = Undefined,
-    sa_type: Type[Any],
+    sa_type: Type[Any] = Undefined,
     sa_column: Union[Column, UndefinedType] = Undefined,  # type: ignore
     sa_column_args: Union[Sequence[Any], UndefinedType] = Undefined,
     sa_column_kwargs: Union[Mapping[str, Any], UndefinedType] = Undefined,