diff --git a/src/ptr_stack.h b/src/ptr_stack.h
index 4c6af11..03dd364 100644
--- a/src/ptr_stack.h
+++ b/src/ptr_stack.h
@@ -6,6 +6,7 @@
 #endif
 
 #include "yaml-cpp/noncopyable.h"
+#include <cstddef>
 #include <cstdlib>
 #include <memory>
 #include <vector>
diff --git a/src/ptr_vector.h b/src/ptr_vector.h
index aa103b8..d1742a3 100644
--- a/src/ptr_vector.h
+++ b/src/ptr_vector.h
@@ -6,6 +6,7 @@
 #endif
 
 #include "yaml-cpp/noncopyable.h"
+#include <cstddef>
 #include <cstdlib>
 #include <memory>
 #include <vector>