diff --git a/src/cargopacket.cpp b/src/cargopacket.cpp
index a09a978a11..e97ff866c9 100644
--- a/src/cargopacket.cpp
+++ b/src/cargopacket.cpp
@@ -37,7 +37,7 @@ CargoPacket::~CargoPacket()
 	this->count = 0;
 }
 
-bool CargoPacket::SameSource(CargoPacket *cp)
+bool CargoPacket::SameSource(const CargoPacket *cp) const
 {
 	return this->source_xy == cp->source_xy && this->days_in_transit == cp->days_in_transit && this->paid_for == cp->paid_for;
 }
diff --git a/src/cargopacket.h b/src/cargopacket.h
index 0468572358..866b41ad0e 100644
--- a/src/cargopacket.h
+++ b/src/cargopacket.h
@@ -53,7 +53,7 @@ struct CargoPacket : PoolItem<CargoPacket, CargoPacketID, &_CargoPacket_pool> {
 	 * @param cp the cargo packet to compare to
 	 * @return true if and only if days_in_transit and source_xy are equal
 	 */
-	bool SameSource(CargoPacket *cp);
+	bool SameSource(const CargoPacket *cp) const;
 };
 
 /**