diff --git a/src/count.hpp b/src/count.hpp
index 3e18e20..f231f26 100644
--- a/src/count.hpp
+++ b/src/count.hpp
@@ -4,6 +4,7 @@
 #include "data_types.hpp"
 
 #include <algorithm>
+#include <cstddef>
 #include <functional>
 
 namespace phosphor