diff --git a/proto/ei-scanner b/proto/ei-scanner
index b68c12c..ab067f8 100755
--- a/proto/ei-scanner
+++ b/proto/ei-scanner
@@ -305,7 +305,7 @@ class Interface:
     events: List[Event] = attr.ib(init=False, factory=list)
     enums: List[Enum] = attr.ib(init=False, factory=list)
 
-    mode: str = attr.ib()  # "ei" or "eis"
+    mode: str = attr.ib(validator=attr.validators.in_(["ei", "eis", "brei"]))
     description: Optional[Description] = attr.ib(default=None)
 
     @property