diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 25254cb1..292754a8 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -31,7 +31,7 @@ jobs:
                 curl -L -O https://tiker.net/ci-support-v0
                 . ./ci-support-v0
                 build_py_project_in_conda_env
-                cipip install pytest modepy optype
+                cipip install pytest modepy 'optype<0.18'
                 cipip install basedpyright
                 basedpyright
 
diff --git a/pyproject.toml b/pyproject.toml
index 5610745a..5906b6c1 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -54,7 +54,9 @@ doc = [
 test = [
     "basedpyright",
     "pytest",
-    "ruff"
+    "ruff",
+    # NOTE: 0.18 requires Python 3.12
+    "optype<0.18",
 ]
 
 [project.urls]