diff --git a/common/ngram-mod.h b/common/ngram-mod.h
index cf3c89c91..7af92e9dd 100644
--- a/common/ngram-mod.h
+++ b/common/ngram-mod.h
@@ -2,6 +2,7 @@
 
 #include <cstdint>
 #include <vector>
+#include <cstddef>
 
 //
 // common_ngram_mod