diff --git a/src/libcharon/plugins/vici/vici_socket.h b/src/libcharon/plugins/vici/vici_socket.h
index 872783665..e82d43dd7 100644
--- a/src/libcharon/plugins/vici/vici_socket.h
+++ b/src/libcharon/plugins/vici/vici_socket.h
@@ -26,7 +26,9 @@
 /**
  * Maximum size of a single message exchanged.
  */
+#ifndef VICI_MESSAGE_SIZE_MAX
 #define VICI_MESSAGE_SIZE_MAX (512 * 1024)
+#endif
 
 typedef struct vici_socket_t vici_socket_t;