diff --git a/net/ethernet.cc b/net/ethernet.cc
index 66e9744b0e..2e9d77c8cd 100644
--- a/net/ethernet.cc
+++ b/net/ethernet.cc
@@ -8,8 +8,9 @@ namespace net {
 
 std::ostream& operator<<(std::ostream& os, ethernet_address ea) {
     auto& m = ea.mac;
+    using u = uint32_t;
     return fprint(os, "%02x:%02x:%02x:%02x:%02x:%02x",
-            m[0], m[1], m[2], m[3], m[4], m[5]);
+            u(m[0]), u(m[1]), u(m[2]), u(m[3]), u(m[4]), u(m[5]));
 }
 
 }