diff --git a/Dockerfile b/Dockerfile
index aa25af9c..af3e31e9 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -1,4 +1,4 @@
-FROM eiriktsarpalis/dotnet-sdk-mono:3.1.201-buster
+FROM eiriktsarpalis/dotnet-sdk-mono:3.1.300-buster
 
 # allow pushing docs & tags using docker builds:
 # pass git credentials using $GITHUB_TOKEN environment variable
diff --git a/RELEASE_NOTES.md b/RELEASE_NOTES.md
index ed7ff5f2..a1a8eeec 100644
--- a/RELEASE_NOTES.md
+++ b/RELEASE_NOTES.md
@@ -1,3 +1,6 @@
+### 6.1.1
+* Fix CustomAssignmentOrSpacedAttribute interop with optional fields.
+
 ### 6.1.0
 * Add a CustomAssignmentOrSpacedAttribute for custom assignments that can also be space separated.