diff --git a/lib/md4.c b/lib/md4.c
index 067c211e42..58dd1166cf 100644
--- a/lib/md4.c
+++ b/lib/md4.c
@@ -28,6 +28,7 @@
 
 #include <string.h>
 
+#include "strdup.h"
 #include "curl_md4.h"
 #include "warnless.h"