diff --git a/panda/src/display/Sources.pp b/panda/src/display/Sources.pp
index fd8688c6a6..f20583da45 100644
--- a/panda/src/display/Sources.pp
+++ b/panda/src/display/Sources.pp
@@ -87,6 +87,7 @@
     graphicsStateGuardian.I \
     graphicsStateGuardian.h \
     graphicsWindow.I graphicsWindow.h \
+    graphicsWindowProc.h \
     graphicsThreadingModel.I graphicsThreadingModel.h \
     graphicsWindowInputDevice.I graphicsWindowInputDevice.h \
     graphicsDevice.I graphicsDevice.h \