diff --git a/re2/prog.h b/re2/prog.h
index d0889bd..801f8b9 100644
--- a/re2/prog.h
+++ b/re2/prog.h
@@ -11,6 +11,7 @@
 
 #include <stdint.h>
 
+#include <cstring>
 #include <functional>
 #include <string>
 #include <type_traits>