diff --git a/src/legendoptics/pyg4utils.py b/src/legendoptics/pyg4utils.py
index 14a74c6..248449a 100644
--- a/src/legendoptics/pyg4utils.py
+++ b/src/legendoptics/pyg4utils.py
@@ -96,7 +96,8 @@ def _val_pint_to_gdml(v):
         v = v.m_as(base_unit)
         return unit, v
 
-    length_u = ["m", "cm", "mm", "um"]
+    # Only as of Geant4 11.1.0, `um` and `nm` are supported.
+    length_u = ["km", "m", "cm", "mm", "um", "nm"]
     dimless_props = [
         "RINDEX",
         "WLSCOMPONENT",