diff --git a/pyproject.toml b/pyproject.toml
index 7a71a986..9cb1434f 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -214,6 +214,7 @@ env = [
     "PYTHONDEVMODE=1",  # https://docs.python.org/3/library/devmode.html
 ]
 filterwarnings = [
+    "error",
     "always::DeprecationWarning",
     # https://docs.pytest.org/en/latest/how-to/failures.html#warning-about-unraisable-exceptions-and-unhandled-thread-exceptions
     "error::pytest.PytestUnraisableExceptionWarning",