diff --git a/src/window_type.h b/src/window_type.h
--- a/src/window_type.h
+++ b/src/window_type.h
@@ -107,6 +107,7 @@ enum WindowClass {
 	WC_AI_LIST,
 	WC_AI_SETTINGS,
 	WC_NEWGRF_INSPECT,
+	WC_SPRITE_ALIGNER,
 
 	WC_INVALID = 0xFFFF
 };