diff --git a/openconnect-internal.h b/openconnect-internal.h
index 732d9360..5163e41e 100644
--- a/openconnect-internal.h
+++ b/openconnect-internal.h
@@ -73,6 +73,7 @@
 #include <sys/time.h>
 #include <sys/types.h>
 #include <unistd.h>
+#include <string.h>
 
 #ifdef LIBPROXY_HDR
 #include LIBPROXY_HDR