diff --git a/src/ifcopenshell-python/ifcopenshell/util/unit.py b/src/ifcopenshell-python/ifcopenshell/util/unit.py
index a218a9504f..102b7b0d61 100644
--- a/src/ifcopenshell-python/ifcopenshell/util/unit.py
+++ b/src/ifcopenshell-python/ifcopenshell/util/unit.py
@@ -604,8 +604,8 @@ def format_length(
     decimal_places: int = 2,
     suppress_zero_inches=True,
     unit_system: Literal["metric", "imperial"] = "imperial",
-    input_unit="foot",
-    output_unit="foot",
+    input_unit: Literal["foot", "inch"] = "foot",
+    output_unit: Literal["foot", "inch"] = "foot",
 ) -> str:
     """Formats a length for readability and imperial formatting