diff --git a/src/ifcopenshell-python/pyproject.toml b/src/ifcopenshell-python/pyproject.toml
index 288e3e3585..9ce2ea39ba 100644
--- a/src/ifcopenshell-python/pyproject.toml
+++ b/src/ifcopenshell-python/pyproject.toml
@@ -27,11 +27,13 @@ dependencies = [
 
 [project.optional-dependencies]
 advanced = [
+  # `express` module - only for generating schemas.
   "networkx",
 ]
 dev = [
   "pytest",
   "tabulate",
+  "types-networkx",
 ]
 
 [project.urls]