diff --git a/src/lang/english.txt b/src/lang/english.txt
--- a/src/lang/english.txt
+++ b/src/lang/english.txt
@@ -2520,6 +2520,7 @@ STR_BROKEN_VEHICLE_LENGTH               
 STR_NEWGRF_BUGGY                                                :{WHITE}NewGRF '{0:RAW_STRING}' provides incorrect information
 STR_NEWGRF_BUGGY_ARTICULATED_CARGO                              :{WHITE}Cargo/refit information for '{1:ENGINE}' differs from purchase list after construction. This might cause autorenew/-replace to fail refitting correctly
 STR_NEWGRF_BUGGY_ENDLESS_PRODUCTION_CALLBACK                    :{WHITE}'{1:STRING}' caused an endless loop in the production callback
+STR_NEWGRF_BUGGY_UNKNOWN_CALLBACK_RESULT                        :{WHITE}Callback 0x{HEX} returned unknown/invalid result 0x{HEX}
 
 # 'User removed essential NewGRFs'-placeholders for stuff without specs
 STR_NEWGRF_INVALID_CARGO                                        :<invalid cargo>