diff --git a/backend/app/db/models/epc_property.py b/backend/app/db/models/epc_property.py
index f04741f8..49901644 100644
--- a/backend/app/db/models/epc_property.py
+++ b/backend/app/db/models/epc_property.py
@@ -607,7 +607,7 @@ class EpcWindowModel(SQLModel, table=True):
     frame_factor: Optional[float] = Field(default=None)
     permanent_shutters_insulated: Optional[str] = Field(default=None)
     transmission_u_value: Optional[float] = Field(default=None)
-    transmission_data_source: Optional[int] = Field(default=None)
+    transmission_data_source: Optional[str] = Field(default=None)
     transmission_solar_transmittance: Optional[float] = Field(default=None)
 
     @classmethod
diff --git a/datatypes/epc/domain/epc_property_data.py b/datatypes/epc/domain/epc_property_data.py
index 8f949b0f..6e6f73a7 100644
--- a/datatypes/epc/domain/epc_property_data.py
+++ b/datatypes/epc/domain/epc_property_data.py
@@ -89,7 +89,7 @@ class SapVentilation:
 @dataclass
 class WindowTransmissionDetails:
     u_value: float
-    data_source: int
+    data_source: Union[int, str]
     solar_transmittance: float