diff --git a/tests/constraints.txt b/tests/constraints.txt
index ea8752f2a0..8afbb29b53 100644
--- a/tests/constraints.txt
+++ b/tests/constraints.txt
@@ -12,6 +12,7 @@ uproot==4.1.1
 iminuit==2.4.0
 # tensorflow
 tensorflow==2.3.1  # tensorflow-probability v0.11.0 requires tensorflow>=2.3
+protobuf<=3.20.1  # protobuf v4.21.0 breaks ABI compatability
 tensorflow-probability==0.11.0  # c.f. PR #1657
 # torch
 torch==1.10.0