diff --git a/src/vector/backends/awkward.py b/src/vector/backends/awkward.py
index 767e3e0d..ef694dc9 100644
--- a/src/vector/backends/awkward.py
+++ b/src/vector/backends/awkward.py
@@ -584,7 +584,10 @@ def elements(self) -> tuple[ArrayOrRecord]:
         return (self.tau,)
 
 
-def _class_to_name(cls: type[VectorProtocol]) -> str:
+T = typing.TypeVar("T", bound=VectorProtocol)
+
+
+def _class_to_name(cls: type[T]) -> str:
     # respect the type of classes inheriting VectorAwkward classes
     is_vector = "vector.backends" in cls.__module__
     if issubclass(cls, Momentum):