diff --git a/src/window_type.h b/src/window_type.h
--- a/src/window_type.h
+++ b/src/window_type.h
@@ -110,6 +110,7 @@ enum WindowClass {
 	WC_SPRITE_ALIGNER,
 	WC_INDUSTRY_CARGOES,
 	WC_GRF_PARAMETERS,
+	WC_BUILD_OBJECT,
 
 	WC_INVALID = 0xFFFF
 };