diff --git a/testing/constraints-3.6.txt b/testing/constraints-3.6.txt
index d2ae7c5d..852feeef 100644
--- a/testing/constraints-3.6.txt
+++ b/testing/constraints-3.6.txt
@@ -6,3 +6,4 @@
 # e.g., if setup.py has "foo >= 1.14.0, < 2.0.0dev",
 # Then this file should have foo==1.14.0
 google-api-core==1.22.2
+protobuf==3.19.0