diff --git a/setup.py b/setup.py
index ff1cc2d..523c108 100644
--- a/setup.py
+++ b/setup.py
@@ -18,5 +18,5 @@
         "License :: OSI Approved :: Apache Software License",
     ],
     packages=["hyperpyyaml"],
-    install_requires=["pyyaml>=5.1", "ruamel.yaml>=0.17.28"],
+    install_requires=["pyyaml>=5.1", "ruamel.yaml>=0.17.28,<0.19.0"],
 )