diff --git a/src/ifcopenshell-python/pyproject.toml b/src/ifcopenshell-python/pyproject.toml
index c15b102587..76d5a588ba 100644
--- a/src/ifcopenshell-python/pyproject.toml
+++ b/src/ifcopenshell-python/pyproject.toml
@@ -25,6 +25,8 @@ dependencies = [
 ]
 
 [project.optional-dependencies]
+# mathutils is broken on Python < 3.10.
+# See: https://gitlab.com/ideasman42/blender-mathutils/-/merge_requests/3
 geometry = ["mathutils"]
 dev = ["pytest"]