diff --git a/src/window_type.h b/src/window_type.h
--- a/src/window_type.h
+++ b/src/window_type.h
@@ -111,6 +111,7 @@ enum WindowClass {
 	WC_INDUSTRY_CARGOES,
 	WC_GRF_PARAMETERS,
 	WC_BUILD_OBJECT,
+	WC_NEWGRF_README,
 
 	WC_INVALID = 0xFFFF
 };