diff --git a/explorer/interpreter/type_utils.h b/explorer/interpreter/type_utils.h
index ca2bcd0129c6..9c3d6de85ed0 100644
--- a/explorer/interpreter/type_utils.h
+++ b/explorer/interpreter/type_utils.h
@@ -5,6 +5,8 @@
 #ifndef CARBON_EXPLORER_INTERPRETER_TYPE_UTILS_H_
 #define CARBON_EXPLORER_INTERPRETER_TYPE_UTILS_H_
 
+#include <cstddef>
+
 #include "explorer/base/nonnull.h"
 
 namespace Carbon {