diff --git a/pyproject.toml b/pyproject.toml
index 5bdc162905..c237c5e6bf 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -63,7 +63,7 @@ matplotlib = "^3.9"
 pineappl = "^1.0.0"
 pandas = "*"
 numpy = "*"
-"ruamel.yaml" = "*"
+"ruamel.yaml" = {version = ">=0.19.1", extras=["oldlibyaml"]}
 validobj = "*"
 prompt_toolkit = "*"
 reportengine = ">=0.32"