diff --git a/DigitalOcean.API.Tests/DigitalOcean.API.Tests.csproj b/DigitalOcean.API.Tests/DigitalOcean.API.Tests.csproj
index f83c358..cd31923 100644
--- a/DigitalOcean.API.Tests/DigitalOcean.API.Tests.csproj
+++ b/DigitalOcean.API.Tests/DigitalOcean.API.Tests.csproj
@@ -1,9 +1,9 @@
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+<Project Sdk="Microsoft.NET.Sdk">
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+    <PackageReference Include="NSubstitute" Version="5.1.0" />
     <PackageReference Include="xunit" Version="2.4.1" />
     <PackageReference Include="xunit.runner.visualstudio" Version="2.4.1">
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