diff --git a/Source/Core/BuilderMono.csproj b/Source/Core/BuilderMono.csproj
index c1a515c8..9462055f 100644
--- a/Source/Core/BuilderMono.csproj
+++ b/Source/Core/BuilderMono.csproj
@@ -244,6 +244,7 @@
     <Compile Include="Rendering\Vector3.cs" />
     <Compile Include="Rendering\Vector4.cs" />
     <Compile Include="Rendering\VertexBuffer.cs" />
+    <Compile Include="Rendering\VisualSlopeHandle.cs" />
     <Compile Include="VisualModes\VisualSlope.cs" />
     <Compile Include="Windows\ThingStatisticsForm.cs">
       <SubType>Form</SubType>