diff --git a/src/subsidy.cpp b/src/subsidy.cpp
--- a/src/subsidy.cpp
+++ b/src/subsidy.cpp
@@ -286,7 +286,7 @@ no_add:;
 		InvalidateWindow(WC_SUBSIDIES_LIST, 0);
 }
 
-bool CheckSubsidised(Station *from, Station *to, CargoID cargo_type)
+bool CheckSubsidised(const Station *from, const Station *to, CargoID cargo_type)
 {
 	Subsidy *s;
 	TileIndex xy;
diff --git a/src/subsidy_func.h b/src/subsidy_func.h
--- a/src/subsidy_func.h
+++ b/src/subsidy_func.h
@@ -16,7 +16,7 @@ Pair SetupSubsidyDecodeParam(const Subsi
 void DeleteSubsidyWithTown(TownID index);
 void DeleteSubsidyWithIndustry(IndustryID index);
 void DeleteSubsidyWithStation(StationID index);
-bool CheckSubsidised(Station *from, Station *to, CargoID cargo_type);
+bool CheckSubsidised(const Station *from, const Station *to, CargoID cargo_type);
 void SubsidyMonthlyHandler();
 
 #endif /* SUBSIDY_FUNC_H */