@@ -415,6 +415,7 @@ def get_sqlachemy_type(field: ModelField) -> Any:
         return AutoString
     if issubclass(field.type_, uuid.UUID):
         return GUID
+    raise ValueError(f"The field {field.name} has no matching SQLAlchemy type")
 
 
 def get_column_from_field(field: ModelField) -> Column:  # type: ignore
 
@@ -0,0 +1,21 @@
+from typing import Optional
+
+import pytest
+from sqlmodel import Field, SQLModel
+
+
+def test_missing_sql_type():
+    class CustomType:
+        @classmethod
+        def __get_validators__(cls):
+            yield cls.validate
+
+        @classmethod
+        def validate(cls, v):
+            return v
+
+    with pytest.raises(ValueError):
+
+        class Item(SQLModel, table=True):
+            id: Optional[int] = Field(default=None, primary_key=True)
+            item: CustomType