diff --git a/backprop/prompt.h b/backprop/prompt.h
index 6f762dc..76acb56 100644
--- a/backprop/prompt.h
+++ b/backprop/prompt.h
@@ -16,6 +16,7 @@
 #ifndef THIRD_PARTY_GEMMA_CPP_GEMMA_PROMPT_H_
 #define THIRD_PARTY_GEMMA_CPP_GEMMA_PROMPT_H_
 
+#include <stddef.h>
 #include <vector>
 
 namespace gcpp {