diff --git a/src/AWSEncryptionSDK.csproj b/src/AWSEncryptionSDK.csproj
index 4ace3950b..7cf7723ea 100644
--- a/src/AWSEncryptionSDK.csproj
+++ b/src/AWSEncryptionSDK.csproj
@@ -50,6 +50,11 @@
     <PackageReference Include="AWSSDK.Core" Version="3.3.104.1" />
     <PackageReference Include="AWSSDK.KeyManagementService" Version="3.3.103" />
     <PackageReference Include="Portable.BouncyCastle" Version="1.8.5.2" />
+    <!--
+      System.Collections.Immutable can be removed once dafny.msbuild is updated with
+      https://github.com/dafny-lang/dafny.msbuild/pull/10 and versioned
+    -->
+    <PackageReference Include="System.Collections.Immutable" Version="1.7.0" />
   </ItemGroup>
 
   <PropertyGroup>