diff --git a/src/window_type.h b/src/window_type.h
--- a/src/window_type.h
+++ b/src/window_type.h
@@ -109,6 +109,7 @@ enum WindowClass {
 	WC_NEWGRF_INSPECT,
 	WC_SPRITE_ALIGNER,
 	WC_INDUSTRY_CARGOES,
+	WC_GRF_PARAMETERS,
 
 	WC_INVALID = 0xFFFF
 };