diff --git a/kokoro/linux/dockerfile/test/csharp/Dockerfile b/kokoro/linux/dockerfile/test/csharp/Dockerfile
index 13b788cb6f19..1a27206e1cb7 100644
--- a/kokoro/linux/dockerfile/test/csharp/Dockerfile
+++ b/kokoro/linux/dockerfile/test/csharp/Dockerfile
@@ -1,4 +1,4 @@
-FROM debian:bullseye
+FROM debian:buster
 
 # Install dependencies.  We start with the basic ones require to build protoc
 # and the C++ build
@@ -22,6 +22,10 @@ RUN apt-get update && apt-get install -y \
   wget \
   && apt-get clean
 
+# Update ca-certificates to fix known buster + .NET 5 issue
+# https://github.com/NuGet/Announcements/issues/49
+RUN apt-get update && apt-get install -y ca-certificates && apt-get clean
+
 # dotnet SDK prerequisites
 RUN apt-get update && apt-get install -y libunwind8 libicu57 && apt-get clean