diff --git a/core/fxcrt/fx_unicode.cpp b/core/fxcrt/fx_unicode.cpp
index 2983655..9b69e98 100644
--- a/core/fxcrt/fx_unicode.cpp
+++ b/core/fxcrt/fx_unicode.cpp
@@ -6,6 +6,8 @@
 
 #include "core/fxcrt/fx_unicode.h"
 
+#include <stddef.h>
+
 #include "third_party/base/check.h"
 #include "third_party/base/cxx17_backports.h"
 
diff --git a/third_party/base/cxx17_backports.h b/third_party/base/cxx17_backports.h
index de1ab07..1df15f2 100644
--- a/third_party/base/cxx17_backports.h
+++ b/third_party/base/cxx17_backports.h
@@ -5,6 +5,8 @@
 #ifndef THIRD_PARTY_BASE_CXX17_BACKPORTS_H_
 #define THIRD_PARTY_BASE_CXX17_BACKPORTS_H_
 
+#include <stddef.h>
+
 namespace pdfium {
 
 // C++14 implementation of C++17's std::size():