diff --git a/include/pixel/RgbaColor.h b/include/pixel/RgbaColor.h
index 659e7d2..6475674 100644
--- a/include/pixel/RgbaColor.h
+++ b/include/pixel/RgbaColor.h
@@ -9,6 +9,7 @@
 #ifndef LIBPIXEL_RGBACOLOR_H
 #define LIBPIXEL_RGBACOLOR_H
 
+#include <cstddef>
 #include <cstdint>
 
 namespace pixel {
diff --git a/include/pixel/RgbaImage.h b/include/pixel/RgbaImage.h
index 1b900fe..60646b4 100644
--- a/include/pixel/RgbaImage.h
+++ b/include/pixel/RgbaImage.h
@@ -11,6 +11,7 @@
 
 #include <pixel/RgbaColor.h>
 
+#include <cstddef>
 #include <cstdint>
 #include <memory>