diff --git a/Source/Core/Util/EncounterSlot.cpp b/Source/Core/Util/EncounterSlot.cpp
index adddbdab..71c98e83 100644
--- a/Source/Core/Util/EncounterSlot.cpp
+++ b/Source/Core/Util/EncounterSlot.cpp
@@ -20,6 +20,7 @@
 #include "EncounterSlot.hpp"
 #include <Core/Enum/Encounter.hpp>
 #include <array>
+#include <cstddef>
 
 namespace
 {