diff --git a/requirements.txt b/requirements.txt
index 48c448b27c767dbf526dd3a2cf5f9a10060c8213..d9a0795ba5ad1d9b27e605ba02e8b94b0ceb44b9 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -10,4 +10,4 @@ uproot>=4.0.0
 awkward>=1.0.0
 pandas
 mendeleev
-proposal
+proposal==6.1.6