diff --git a/src/window_type.h b/src/window_type.h
--- a/src/window_type.h
+++ b/src/window_type.h
@@ -106,6 +106,7 @@ enum WindowClass {
 	WC_AI_DEBUG,
 	WC_AI_LIST,
 	WC_AI_SETTINGS,
+	WC_NEWGRF_INSPECT,
 
 	WC_INVALID = 0xFFFF
 };