diff --git a/src/newgrf_commons.h b/src/newgrf_commons.h
--- a/src/newgrf_commons.h
+++ b/src/newgrf_commons.h
@@ -300,6 +300,8 @@ uint32 GetNearbyTileInformation(TileInde
 uint32 GetCompanyInfo(CompanyID owner, const struct Livery *l = NULL);
 CommandCost GetErrorMessageFromLocationCallbackResult(uint16 cb_res, uint32 grfid, StringID default_error);
 
+void ErrorUnknownCallbackResult(uint32 grfid, uint16 cbid, uint16 cb_res);
+
 /**
  * Data related to the handling of grf files.
  * @tparam Tcnt Number of spritegroups