diff --git a/pyproject.toml b/pyproject.toml
index 8ff4765..8d1fb92 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -31,7 +31,8 @@ classifiers = [
 ]
 requires-python = ">=3.9"
 dependencies = [
-    "numpy",
+    "numpy<2.0 ; python_version <= '3.9'",
+    "numpy ; python_version >= '3.10'",
     "pint",
     "pyg4ometry",
     "pylegendmeta>=v0.9.0a2",
diff --git a/src/legendoptics/pyg4utils.py b/src/legendoptics/pyg4utils.py
index f04990c..3a03776 100644
--- a/src/legendoptics/pyg4utils.py
+++ b/src/legendoptics/pyg4utils.py
@@ -62,8 +62,8 @@ def pyg4_def_scint_by_particle_type(mat, scint_cfg: ScintConfig) -> None:
 
 @pint.register_unit_format("gdml")
 def _gdml_format(unit, registry, **options):
-    proc = {u.replace("µ", "u"): e for u, e in unit.items()}
-    return pint.formatter(
+    proc = {ureg._get_symbol(u).replace("µ", "u"): e for u, e in unit.items()}
+    return pint.formatting.formatter(
         proc.items(),
         as_ratio=True,
         single_denominator=False,
@@ -89,8 +89,11 @@ def _val_pint_to_gdml(v):
 
         base_unit = v.units
 
-        unit = f"{base_unit:~gdml}"
+        # TODO: drop the extra check for dimensionless quantities after dropping support for pint <= 0.23
+        unit = "" if v.check("1") else f"{base_unit:gdml}"
         assert unit == f"{base_unit:~}".replace(" ", "").replace("µ", "u")
+        assert "dimensionless" not in unit
+
         msg = f"Unit pint->gdml: {unit} - {base_unit}"
         log.debug(msg)