diff --git a/pyproject.toml b/pyproject.toml
index c2f3711e..da53a6c7 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -33,7 +33,7 @@ dev = [
     "opencv-python-headless>=4.5.4.58",
     "streamlit-session-memo>=0.3.2",
     # For testing older versions of Streamlit: https://discuss.streamlit.io/t/modulenotfounderror-no-module-named-altair-vegalite-v4/42921
-    "altair<5",
+    "altair<6",
     # For testing older versions of Streamlit: https://discuss.streamlit.io/t/streamlit-run-with-protobuf-error/25632/3
     "protobuf<=3.20",
 ]