diff --git a/src/dask_awkward/layers/layers.py b/src/dask_awkward/layers/layers.py
index 244e758df..86eca7ada 100644
--- a/src/dask_awkward/layers/layers.py
+++ b/src/dask_awkward/layers/layers.py
@@ -93,7 +93,7 @@ def mock(self) -> AwkwardArray:
         assert self._meta is not None
         return self._meta
 
-    def mock_empty(self, backend: str = "cpu") -> AwkwardArray:
+    def mock_empty(self, backend: BackendT = "cpu") -> AwkwardArray:
         import awkward as ak
 
         if backend not in ("cpu", "jax", "cuda"):