diff --git a/src/toil/wdl/wdltoil.py b/src/toil/wdl/wdltoil.py
index 72a8a3724a..2c7e155b23 100755
--- a/src/toil/wdl/wdltoil.py
+++ b/src/toil/wdl/wdltoil.py
@@ -3558,7 +3558,7 @@ def main() -> None:
                 # have to cast from more specific to less specific ones here.
                 # The miniwld values_from_json function can evaluate
                 # expressions in the inputs or something.
-                WDLTypeDeclBindings = WDL.Env.Bindings[WDL.Tree.Decl] | WDL.Env.Bindings[WDL.Type.Base]
+                WDLTypeDeclBindings = Union[WDL.Env.Bindings[WDL.Tree.Decl], WDL.Env.Bindings[WDL.Type.Base]]
                 input_bindings = WDL.values_from_json(
                     inputs,
                     cast(WDLTypeDeclBindings, target.available_inputs),