diff --git a/Source/Core/BuilderMono.csproj b/Source/Core/BuilderMono.csproj
index d98a55d9..e0518643 100644
--- a/Source/Core/BuilderMono.csproj
+++ b/Source/Core/BuilderMono.csproj
@@ -539,7 +539,6 @@
     <Compile Include="IO\Lump.cs" />
     <Compile Include="IO\MapSetIO.cs" />
     <Compile Include="IO\UniversalMapSetIO.cs" />
-    <Compile Include="IO\UnknownImageReader.cs" />
     <Compile Include="IO\WAD.cs" />
     <Compile Include="Windows\MainForm.cs">
       <SubType>Form</SubType>