diff --git a/OpenRa.TechTreeTest/OpenRa.TechTreeTest.csproj b/OpenRa.TechTreeTest/OpenRa.TechTreeTest.csproj
index d6c63f1a6a..260fc5a70c 100644
--- a/OpenRa.TechTreeTest/OpenRa.TechTreeTest.csproj
+++ b/OpenRa.TechTreeTest/OpenRa.TechTreeTest.csproj
@@ -36,6 +36,7 @@
     <Reference Include="System.Xml" />
   </ItemGroup>
   <ItemGroup>
+    <Compile Include="Building.cs" />
     <Compile Include="Form1.cs">
       <SubType>Form</SubType>
     </Compile>