diff --git a/Wox.Test/Wox.Test.csproj b/Wox.Test/Wox.Test.csproj
index 8337d59191..385fe023e5 100644
--- a/Wox.Test/Wox.Test.csproj
+++ b/Wox.Test/Wox.Test.csproj
@@ -45,6 +45,7 @@
     <Reference Include="nunit.framework, Version=3.12.0.0, Culture=neutral, PublicKeyToken=2638cd05610744eb, processorArchitecture=MSIL">
       <HintPath>..\packages\NUnit.3.12.0\lib\net45\nunit.framework.dll</HintPath>
     </Reference>
+    <Reference Include="System" />
     <Reference Include="System.Core" />
   </ItemGroup>
   <ItemGroup>