diff --git a/src/include/lwip/udp.h b/src/include/lwip/udp.h
index 6d674569..9d27d0d9 100644
--- a/src/include/lwip/udp.h
+++ b/src/include/lwip/udp.h
@@ -49,9 +49,9 @@ struct udp_hdr {
   PACK_STRUCT_FIELD(u16_t chksum);
 } PACK_STRUCT_STRUCT;
 
-#define UDP_FLAGS_NOCHKSUM 0x01
-#define UDP_FLAGS_UDPLITE  0x02
-#define UDP_FLAGS_CONNECTED  0x04
+#define UDP_FLAGS_NOCHKSUM 0x01U
+#define UDP_FLAGS_UDPLITE  0x02U
+#define UDP_FLAGS_CONNECTED  0x04U
 
 struct udp_pcb {
   struct udp_pcb *next;