diff --git a/cg/services/order_validation_service/workflows/rna_fusion/models/case.py b/cg/services/order_validation_service/workflows/rna_fusion/models/case.py
index af9e7d8a3a..ebf9ded595 100644
--- a/cg/services/order_validation_service/workflows/rna_fusion/models/case.py
+++ b/cg/services/order_validation_service/workflows/rna_fusion/models/case.py
@@ -1,4 +1,4 @@
-from pydantic import Discriminator, Tag
+from pydantic import Discriminator, Field, Tag
 from typing_extensions import Annotated
 
 from cg.services.order_validation_service.models.case import Case
@@ -13,4 +13,6 @@
 class RnaFusionCase(Case):
     cohorts: list[str] | None = None
     synopsis: str | None = None
-    samples: list[Annotated[NewSample | OldSample, Discriminator(has_internal_id)]]
+    samples: list[Annotated[NewSample | OldSample, Discriminator(has_internal_id)]] = Field(
+        max_length=1
+    )