diff --git a/src/newgrf_commons.cpp b/src/newgrf_commons.cpp
index a72d5a9ccd..4c8dcf8f57 100644
--- a/src/newgrf_commons.cpp
+++ b/src/newgrf_commons.cpp
@@ -521,7 +521,7 @@ void ErrorUnknownCallbackResult(uint32_t grfid, uint16_t cbid, uint16_t cb_res)
 	/* debug output */
 	Debug(grf, 0, "{}", StrMakeValid(GetString(STR_NEWGRF_BUGGY, grfconfig->GetName())));
 
-	Debug(grf, 0, "{}", StrMakeValid(GetString(STR_NEWGRF_BUGGY_UNKNOWN_CALLBACK_RESULT, cbid, cb_res)));
+	Debug(grf, 0, "{}", StrMakeValid(GetString(STR_NEWGRF_BUGGY_UNKNOWN_CALLBACK_RESULT, std::monostate{}, cbid, cb_res)));
 }
 
 /**