diff --git a/libopenage/pathfinding/cost_field.cpp b/libopenage/pathfinding/cost_field.cpp
index 6c29e24f59..92afea4b9b 100644
--- a/libopenage/pathfinding/cost_field.cpp
+++ b/libopenage/pathfinding/cost_field.cpp
@@ -57,24 +57,25 @@ void CostField::set_costs(std::vector<cost_t> &&cells, const time::time_t &valid
 }
 
 bool CostField::stamp(size_t idx, cost_t cost, const time::time_t &stamped_at) {
-	if (this->cost_stamps.contains(idx)) return false;
+	if (this->cost_stamps[idx].has_value()) return false;
 
 	cost_t original_cost = this->get_cost(idx);
-	this->cost_stamps[idx].original_cost = original_cost;
-	this->cost_stamps[idx].stamp_time = stamped_at;
+	this->cost_stamps[idx]->original_cost = original_cost;
+	this->cost_stamps[idx]->stamp_time = stamped_at;
 
 	this->set_cost(idx, cost, stamped_at);
 	return true;
 }
 
 bool CostField::unstamp(size_t idx, const time::time_t &unstamped_at) {
-	if (!this->cost_stamps.contains(idx)) return false;
-	if (unstamped_at < this->cost_stamps[idx].stamp_time) return false;
+	if (!this->cost_stamps[idx].has_value()) return false;
+	if (unstamped_at < this->cost_stamps[idx]->stamp_time) return false;
 
-	cost_t original_cost = cost_stamps[idx].original_cost;
+	cost_t original_cost = cost_stamps[idx]->original_cost;
 
 	this->set_cost(idx, original_cost, unstamped_at);
-	return this->cost_stamps.erase(idx) != 0;
+	this->cost_stamps[idx].reset();
+	return true;
 }
 
 bool CostField::is_dirty(const time::time_t &time) const {
diff --git a/libopenage/pathfinding/cost_field.h b/libopenage/pathfinding/cost_field.h
index 3750ed0680..b00a49f6fb 100644
--- a/libopenage/pathfinding/cost_field.h
+++ b/libopenage/pathfinding/cost_field.h
@@ -4,6 +4,7 @@
 
 #include <cstddef>
 #include <vector>
+#include <optional>
 
 #include "pathfinding/types.h"
 #include "time/time.h"