diff --git a/libfastsignals/include/function_detail.h b/libfastsignals/include/function_detail.h
index 3b41170..c01cc95 100644
--- a/libfastsignals/include/function_detail.h
+++ b/libfastsignals/include/function_detail.h
@@ -4,6 +4,7 @@
 #include <cstdint>
 #include <type_traits>
 #include <utility>
+#include <cstddef>
 
 namespace is::signals::detail
 {