diff --git a/scripts/nightly.yaml b/scripts/nightly.yaml
index f0a4bfed0..c93738e58 100644
--- a/scripts/nightly.yaml
+++ b/scripts/nightly.yaml
@@ -11,7 +11,7 @@ stages:
   - job: Mac
     displayName: "Mac Build"
     pool:
-      vmImage: "macOS-latest"
+      vmImage: "macOS-11"
     steps:
     - script: python scripts/mk_unix_dist.py --dotnet-key=$(Build.SourcesDirectory)/resources/z3.snk
     - script: git clone https://github.com/z3prover/z3test z3test
@@ -25,7 +25,7 @@ stages:
   - job: MacArm64
     displayName: "Mac ARM64 Build"
     pool:
-      vmImage: "macOS-latest"
+      vmImage: "macOS-11"
     steps:
     - script: python scripts/mk_unix_dist.py --dotnet-key=$(Build.SourcesDirectory)/resources/z3.snk --arch=arm64 --os=osx-11.0
     - script: git clone https://github.com/z3prover/z3test z3test